Flashcards on Epidemiology Final
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Core functions of public health
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Assessment, Policy Development, Assurance
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Primary Prevention
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Strategies, tactics, and procedures that prevent the of the occurrence of disease in the first place (normal)
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Secondary Prevention
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Strategies, tactics, and procedures, including screening tests, that detect disease as early as possible so that its progress can be arrested and, if possible, the disease eradicated (preclinical)
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Tertiary Prevention
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Strategies, tactics, and procedures, including interventions, that aim to arrest the progress of established disease (clinical)
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Epidemiology
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The study of f the distribution and determinants of health related states or events in specified populations, and the application of this study to control health problems
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Numerators
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"Cases" of disease and death within population (surveillance, surveys, hospital records)
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Denominators
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population at risk over time (census, samples of populations, surveys)
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Proportion
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Division of two related numbers; numerator is subset of denominator
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Incidence
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The number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease; a measure of riks;(Number of NEW cases of a disease occurring in the population during a specified period of time)/ (Number of persons who are AT RISK of developing the disease during that same period of time)
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Prevalence
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The proportion (%) of the population affected by the disease at a given time; not a measure of risk; (Number of existing cases of disease present in the population at a specified time)/(Number of persons in the population at that specified time) x 100
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Prevalence
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Incidence*Duration
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Increases to Prevalence
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Longer duration of the disease; Prolongation of life of patient without cure; Increase in new cases; In migration of cases; Out migration of healthy people; Improvements in diagnosis (or better reporting)
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Decreases to Prevalence
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Shorter duration of disease; High case fatality rate from disease; Decrease in new cases; In migration of healthy people; Out migration of cases; Improved cure rate of cases
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Dynamic Populations
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membership is based on a condition and is transitory
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Annual All Causes Mortality Rate (per 1000)
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(Total # of deaths from all causes in 1 year)/(# of persons in the population at mid year) x1000
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Annual Cause Specific Mortality Rate (per 1000)
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(# of deaths from disease A in 1 year)/(# of persons in the population at mid year)x1000
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Cumulative Incidence Rate
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(# of new cases of a disease occurring in the population @ specific time period)/(# of persons at risk for the disease at the beginning of the period)
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Persons at Risk
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Not at risk include those who have the disease, who lack organ defining disease or who are immune to the disease
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Calculating Person-time at risk
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obtain time at risk for each individual in the population, sum all time at risk
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Incidence in person-time
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# new cases/Person-time at risk
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Math of Calculating Person-time
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Cases/pop/time → (cases/pop)/time → cases/ (pop*time) → #cases/pop-time
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Fixed Populations
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membership based on an event and is permanent
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Crude Rates
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refer to rates calculated for the entire population of interest without regard to different subgroups of the population
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Persons at Risk
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Not at risk include those who have the disease, who lack organ defining disease or who are immune to the disease
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Fixed Populations
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membership based on an event and is permanent
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Proportionate Mortality Ratio (PMR)
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(# of deaths due to a particular disease)/(total # of deaths)
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Case fatility rate (CFF)
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(# of deaths from disease A)/(# of people diagnosed with disease A) Mortality rate is a good index of an incidence rate when case fatality is high and when duration of disease is short
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Specific Rates
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are those calculated for designated subgroups or strata of the population.
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Age adjustment
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A procedure for adjusting rates (e.g. death rates), designed to minimize the effects of differences in age composition when comparing rates for different populations
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Direct Age Adjustment
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(Need to know age distributions of both populations) 1) Calculate rates for each population overall and by age category (Divide the number of events by the population) 2) Identify the standard population 3)Apply all rates to standard population all rates to standard population by age category to obtain expected number of events for each population 4)Sum the expected number of events for each population 5) For each population, divide the sum of expected number of events by the total standard population to obtain the age adjusted rate
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Indirect Age-Adjustment
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(Used when age distribution of population of interest is not known) 1) Identify total observed events in total observed events in population of interest 2)Identify age specific event rates for known population of comparison 3)Apply age specific event rates (from comparison population) to age specific strata of population of interest to obtain expected number of deaths for each age category 4)Sum the the expected number of events for expected number of events for the population of interest 5)Divide the number of observed events by p y the number of expected events to obtain the standardized mortality ratio (SMR)
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Standardized mortality ratio (SMR)
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(observed # of deaths)/(expected number of deaths)
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"Secular trends" (long term variation)
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Changes in disease occurrence over a period longer than a year
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Cyclic changes or periodic fluctuations (e.g., seasonal variation)
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Recurrent alterations in the frequency of disease
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Endemic
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The constant presence of a disease within a given geographical area or population group; May also refer to the usual prevalence of a also refer to the usual prevalence of a given disease within such an area or group
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Epidemic
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The occurrence in a community or region of occurrence in a community or region ofcases of an illness, specific health related behavior behavior or other health, or other health related related events events clearly clearly in excess of normal expectancy; Relative to usual frequency of the disease
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Pandemic
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An epidemic occurring worldwide, or over over a very wide area, crossing a very wide area, crossing international boundaries, and usually affecting a large number of people
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The Age of Pestilence and Famine
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Mortality is high and and fluctuating, low and variable life expectancy fluctuating, low and variable life expectancy (range 20 40 years)
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The Age of Receding Pandemics
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Mortality declines with fewer epidemics, life expectancy increases (from 30 to 50 years), population growth is sustained and begins to be exponential
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The Age of Degenerative and Man Made Diseases
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Mortality continues to decline and approaches continues to decline and approaches stability at low level, chronic diseases replace infectious diseases as the primary causes of death
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Demographic Equation
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(natural change in population size) Births -Deaths ± Migration
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Crude Birth Rate
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(# Live births in year)/(Midyear population)
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General Fertility Rate
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(# Live births in year)/(Midyear female population aged 15 49 years)
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Infant Mortality Rate
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(# deaths among children < 1 year of age 1 year of age during the year) / (# live births during the year)
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Neonatal Mortality Rate
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(# deaths among children < 28 days of age during the year)/ (# live births during the year)
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Passive Surveillance
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Reporting of disease cases seen in health care facilities
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Active Surveillance
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Special search to find disease cases
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Sentinel Surveillance
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Disease specific reporting systems in defined catchment areas
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Real time Surveillance
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Alert public health care practitioners in early phases of outbreak
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Syndromic Surveillance
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Surveillance using health related data that precede diagnosis and signal a sufficient probability of a case or an outbreak to warrant further public health response (supplements existing surveillance methods) google flu trends
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Incubation period
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the time period from infection to symptom onset.
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Latent period
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the time period from infection to becoming infectious. People often improperly call this the incubation period
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Relative Risk
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(probability of disease among exposed)/(probability of disease among unexposed); (attack rate among exposed)/(attack rate among unexposed) If the RR is much greater than 1 then the exposure is highly associated with disease
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Attack Rate
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(# in risk group developing disease)/(# in risk group)
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Odds Ratio
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(# exposed and sick)/ (# exposed)
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Odds of Exposure
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(exposed/total)/(unexposed/total)
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Point Source
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pinpointed to one causes (either time, agent, place, etc)
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Common Source
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typically an environmental contaminant, something that is stable
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Propagated
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people spreading the disease from one person to the next
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Herd Immunity
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The relative resistance of a group to an outbreak of a disease when a critical proportion, but not all, of the group is immune; this occurs when the disease is spread by person to person contact
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Pre clinical
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Post biologic onset, but pre symotoms
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Clinical
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Post symtoms includes diagnosis, therapy, outcome
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Valid and Reliable
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tightly centered (reliable) around the true value (valid)
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Valid
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Centered around true value, but very loosely (large bell curve)
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Reliable
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tightly centered around a single point, but point not the true value (sharp, narrow curve)
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Validity
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An expression of the degree to which a measurement measures what it purports to measure
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Sensitivity
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The measurement ability of the test to identify correctly those who have the disease; (probability of testing positive, given that they actually have disease)=a/(a+c)
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Specificity
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The measurement ability of the test to identify correctly those who do not have the disease; (probability of testing negative, given that there is actually no disease)=d/(b+d)
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Two Stage Screening (sequential)
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Use a less expensive, less invasive, or less uncomfortable test first, rescreen those who screened positive with a more expensive, more invasive, or more uncomfortable test (or the same one); increases specificity (rules out false positives) but decreases sensitivity (true positives might become false negatives)
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Identified with disease (sequential screening)
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anyone who tests positive on either test
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Identified without disease (sequential screening)
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anyone who tests negative on both
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Net sensitivity(sequential screening)
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# identified as positive twice among those with the disease = sensitivity of test 1 x sensitivity of test 2
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Net sensitivity (sequential screening)
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# identified as negative by either test among those without the disease = 1 -[(1-specificity test 1) x (1-specificity test 2 )]
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Positive Predictive Value
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Of those who test positive, the proportion who actually have the disease
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Simultaneous Screening (parallel)
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Apply two screening tests simultaneously (i.e., test everyone twice). Increases sensitivity (rule out false negatives), decreases specificity (true negatives labeled as false positive)
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Identified with disease (Simultaneous Screening)
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anyone who tests positive on either test
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Identified without disease (Simultaneous Screening)
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anyone who tests negative on both
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Net sensitivity
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test positive on both or either test / true disease =(test 1 sensitivity x test 2 sensitivity x true disease) + (Test + on Test 1 only) +(Test + on Test B only)) / (total true disease)
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Test positive on both tests
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test 1 sensitivity x test 2 sensitivity x true disease
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Test positive on one test
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correctly tested positive for that test minus overlap a -((a1 /(a1+c1)) x (a2/(a2+c2) x (a+c))
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Net specificity
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test negative both tests / true no disease
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Test negative on both tests
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test 1 specificity x test 2 specificity x true no disease
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Positive Predictive Value
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Of those who test positive, the proportion who actually have the disease= true positives/test positives=a/(a+b); depends on prevalence of disease and specificity of test
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Negative Predictive Value
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The proportion of patients who test negative who are actually free of the disease = true negatives/test - = d/ (c+d)
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Overall percent agreement (concordance)
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Sum(middle diagonal)/total x100
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Kappa Statistic
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Percent agreement attributable to actual observed agreement
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Number of people living with HIV
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Total: 33.3 million
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People newly infected with HIV
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Total: 2.6 million
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Paths of exposure
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air/food/water, stress, Radiation, Sunlight, vector
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Agents
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Biologic, Chemical, Physical, Nutritional
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Host factors
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age, sex, genetics, personality (risk taking), health status (previous/current disease), immune status, nutritional status, education, behaviors (smoking)
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Environmental factors
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Physical- temperature, crowding, housing, water, sewage, food, radiation, air pollution. Social- support, behavioral modeling (family, peers, culture, media), politics, economics
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Reservoir
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Natural habitat in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host
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Vector
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An insect or any living carrier that transports an infectious agent from an infected individual or its wastes to a susceptible individual or its food or immediate surroundings
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Incubation Period (in infectious diseases)
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The time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question
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Latent Period(in infectious diseases)
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The interval between exposure to infectious agent and the onset of infectiousness for the disease in question (may be shorter or longer than incubation period)
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Chronic Diseases
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Also referred to as non communicable diseases and degenerative diseases Characterized by uncertain etiology, multiple risk factors, long induction period, prolonged course of illness (3 months or longer, according to US National Center for Health Statistics), non contagious
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Temporality
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gauge whether exposures precede outcomes (important, but not apparent, in all studies); Permit direct measurement of changes in outcome over time in an individual
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Prospective Cohort
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Cohort is assembled in the present and followed into the future; obtain exposure information prior to determining endpoint status + implement quality control of measurements; protection against temporal ambiguity, changing technology - time to complete study is longer and more costly
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Retrospective Cohort
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Cohort is assembled in the past and followed forward in time up to the present; obtain endpoint status prior to or at the same time as exposure information + Less expensive, can be done quite rapidly - Dependent on whether data is readily available; complex reconstruction of exposure history; recall bias
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Ambispective/ambidirectional cohort
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combines prospective and retrospective
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Closed cohort
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individuals enter at the same time and only exit if they have end point of interested or are free of the event at the end of the study → does NOT include loss to followup
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Prevalence
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(# of existing cases @ time t)/(Size of study population @ time t)
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Prevalence Ratio
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(Prevalence of disease in exposed/Prevalence of disease in unexposed) = (a/(a+b))/(c/(c+d))
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Prevalence Difference
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( Prevalence of disease in exposed-Prevalence of disease in unexposed)=(a/(a+b)) - (c/(c+d))
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Cumulative Incidence
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# Diseased/Total
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Cumulative Incidence ratio
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(Risk of disease with exposure)/(Risk of disease without exposure)=(a/(# exposed at risk)/(c/(# of unexposed at risk))
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Cumulative incidence difference or Attributable Risk for exposed
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(risk in exposed risk in unexposed) =(a/(a+b)) - (c/(c+d))
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Incidence Rate Ratio
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(disease in exposed/person time at risk in exposed)/(disease in unexposed/person time at risk in unexposed)
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Incidence Rate Difference
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(disease in exposed/person time at risk in exposed) (disease in unexposed/person time at risk in unexposed)
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Ratio measures
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look at the relative difference of the occurrence of disease frequency in exposed population as compared to occurrence of disease frequency in unexposed population (whereas difference measure the absolute difference) *Ratio measures are unitless,
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Primary source population
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defined before disease cases are identified
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Secondary source population
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defined after disease cases are identified
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Primary data
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Collected for the purpose of a study (i.e. Questionnaires)
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Secondary data
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Collected primarily for other purposes (I.e. Medical records)
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Ecologic Fallacy
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The bias that may occur because an association observed between variables on the aggregate/group level does not necessarily represent the association that exists at the individual level. Therefore, we cannot ascribe group characteristics to individual members of the group.
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Prevalence Ratio
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Prevalence in exposed/Prevalence in Unexposed (a/(a+b)/c/(c+d))
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Prevalence Difference
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Prevalence in exposed Prevalence in Unexposed (a/(a+b) c/(c+d))
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Absolute risk
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Incidence of disease
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Background risk
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amount of incidence (risk) that both exposed and non exposed share
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Attributable Risk in Exposed Population
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(incidence attributable to exposure): (Incidence in exposed group) (Incidence in non exposed group)
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Proportion of Incidence Attributable to Exposure In the Exposed Population
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(Incidence in exposed group) (Incidence in non exposed group)/ (Incidence in exposed group)
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Attributable Risk For the Total Population
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Incidence in Total Population - Incidence in non exposed group
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Proportion of Incidence Attributable to Exposure in Total Population
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(Incidence in Total Population) (incidence in non exposed group) / (Incidence in Total Pop)
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Attributable Risk for Total Population (If the incidence in the total population is unknown)
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(Incidence in exposed) x (%exposed in pop) + (Incidence in Non exposed) x (%non exposed in pop)
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Attack Rate
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(# of cases among people at risk) / (total # of people at risk)
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Basic reproductive number (Ro)
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The average number of secondary cases produced by an infective person in a fully susceptible population.
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Effective reproductive number (R)
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The average number of secondary cases produced by an infective person in a population not fully susceptible to infection; If p = proportion successfully immunized, then 1 p is the proportion susceptible; R = Ro (1- p)
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Herd Immunity Threshold
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If R < 1, the infection cannot maintain itself and dies out.
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Critical proportion of the population to be immunized for eradication
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Pc= 1 - (1/Ro)
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Vaccine efficacy
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1 - Relative Risk = 1 (risk in those vaccinated)/(risk in those not vaccinated)
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Confounding by indication
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This bias can arise in observational studies when patients with the worst prognosis are allocated preferentially to a particular treatment; Likely to be systematically different from those not treated, or treated with something else. → fixed by randomization
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Randomization
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Treatment given is not influenced by provider provider bias or patient prognosis bias or patient prognosis avoids "confounding by indication"
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Superiority trials
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one treatment is better than the the other other
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Equivalence or Non inferiority inferiority trials
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trials: two treatments are equivalent (within some small margin); must be "much" smaller than clinically important difference
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Primary Randomization
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to remove the potential for bias in the choice choice of treatment (treatment selection bias of treatment (treatment selection bias, confounding by indication)
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Secondary Randomization
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to increase the likelihood of balance between groups for known and unknown risk factors
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Tertiary Randomization
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to provide a probability basis for statistical testing
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Stratification and randomization
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Ensures balance in treatment assignments within balance in treatment assignments within subgroups defined before Randomization; Stratification variable should be strongly related to outcome; Requires a separate set of treatment assignment schedules for each category of each stratum
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Masking ("Blinding")
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treatment allocation is unknown; reduces bias related to prior knowledge or or beliefs about treatments; for more beliefs about treatments; for more objective
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Cross Over
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people do not always stay within their assigned treatment allocations
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Drop outs
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non adherence to the experimental experimental regimen during regimen during follow up; losses to follow up
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Drop ins
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non adherence to the control control regimen during follow regimen during follow up
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Intention to Treat
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Analyze data based purely on randomization; most conservative analysis for a superiority design, but not most conservative for non inferiority design
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Cross Over Design Clinical Trial
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Each patient serves as his or her own control, thereby ensuring comparability of treatment groups; Feasible only if outcomes are "recurrent" & No "carryover" treatment effect after "washout" period; Generalizability remains an issue
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Factorial Design Clinical Trial
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Potentially economical way to test two treatments simultaneously, if their modes of action are independent; Method to test for treatment synergy to test for treatment synergy (is effect of treatment combination different than expected based on the than expected based on the effects of the treatments alone?)
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Group Allocation Design (cluster randomization) Clinical Trial
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Randomization unit is a group of individuals unit is a group of individuals (community, school, clinic); Individual randomization and intervention is not feasible or is unacceptable (tracking/contamination)
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Relative risk (RR)
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the ratio of rates of bad events between treatment and non treatment groups=Rate in treated/Rate in untreated
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Relative Risk Reduction
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The proportional reduction in rates of bad events between treatment and non treatment groups =(Rate in untreated Rate in treated)/Rate in untreated) =1-RR
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External Validity
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Generalizability from study population to the reference population; May not provide good estimate of the absolute effect of treatment for the entire population; • But ... relative treatment effect may be generalizable to entire population, because qualitatively different treatment responses are rare
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Internal Validity
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within study design, between the two treatment groups; clinical trial provides internally valid estimates of absolute and relative treatment effects
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Disease associations with known genetic diseases
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Down syndrome - Leukemia, Down Syndrome - Alzheimer's disease, Familial adenomatosis - Colon Cancer, Ataxia telangiectasia - Breast Cancer
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adoption studies
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Examine correlation of continuous traits between biological relative pairs and non biological relative pairs; If greater correlation between biological relative pairs, then greater genetic contribution Issues- Adoptees highly selective highly selective, age at adoption varies, Adoptees may retain various levels of contact with biological parents, Adoption records often private
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twin studies
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Monozygotic (MZ) twins share 100% of genetic material; Dizygotic (DZ) twins share, on average, 50% 50% of genetic material of genetic material; If MZ twins are more alike with respect to a disease disease than DZ twins then disease must than DZ twins, then disease must be more "genetic"; Assess Similarities by means of concordance and correlation
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Migrant studies
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Migrants are highly selected; Age at migration varies; Migrants may retain many elements of their their original environment, particularly original environment, particularly those related to culture and lifestyle
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Genetic markers
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A segment of DNA with an identifiable physical location on a chromosome and whose inheritance can be followed
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Incidence Density Sampling
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Risk set is defined at the time of the case ; Individuals who become cases can serve as a control at an earlier time point and can be selected more than once
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Cumulative Incidence Sampling
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Risk set is defined at the end of follow up among those without outcome
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Multiple Controls per Case
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Increases power of the study by increasing sample size; Particularly useful when disease is rare, # cases is low; Beyond 4:1, not much is gained
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Odds of exposure
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(exposed/unexposed)
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Odds Ratio
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(odds of exposure among cases)/(odds of exposure among controls) =(a/c)/(b/d); If the disease incidence is rare (e.g., <10%), then OR ≈ RR
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Inference
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the process of deriving a conclusion from facts or the process of creating a generalization based on observations
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Induction
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to reach a general rule or conclusion by inference from particular facts; rooted in facts
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Cause
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a factor is considered a cause of a disease if it is part of a complex of circumstances in which the frequency of disease is increased by its presence and reduced by its absence
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Risk Factor
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An attribute associated with the occurrence of health-related condition
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Three Essential Characteristics of a Cause
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association, time order, direction
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Association
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a cause must occur together with its effect; a statistical association must exist between a cause and its putative effect.
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Time order
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a cause must precede its effect.
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Direction
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there is an asymmetrical relationship between the cause and effect; a change in the cause produces a change in the effect; changing the effect does not alter the cause.
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Necessary and sufficient
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cause is always present with disease; nothing but cause is needed to result in disease i.e. measles virus and measles
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Necessary and not sufficient
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cause is always present with disease, but disease is not always present with cause i.e. HPV and cervical cancer; tubercle bacillus and TB.
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Necessary Causes
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If a disease is defined by the presence of an agent, that agent is necessary by definition; A necessary cause must always precede the effect i.e. Tuberculosis can only be caused by the tubercle bacillus, however Hepatitis can be caused by many viruses, but Hepatitis C is caused only by the Hepatitis C virus
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Not necessary and sufficient
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disease has multiple causes and can occur without cause of interest i.e. Lung cancer and smoking, radon
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Not necessary and not sufficient
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cause may or may not be present with disease; if cause is present with disease, then some additional factor must also be present i.e. gonorrhea and pelvic inflammatory disease
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Henle-Koch Postulates
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The organism is always found with the disease, the organism is not found with any other disease, the organism, isolated from one who has the disease, and cultured through several generations, produces the disease(in experimental animals)
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Epidemiologic Criteria for Establishing a Cause-Effect Relationship
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Temporal sequence, strength of the association, dose response relationship, consistency of the association, coherence (biologic plausibility), specificity of the association, experiment (cessation of exposure), analogy
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Temporal Sequence
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Study designs that can establish the potential "cause" (risk factor or treatment) precedes the disease include cohort and clinical trials
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No Association Between Exposure, E, and Disease
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iff a/(a+b) = c/(c+d) i.e., RR = 1 OR = 1 Risk difference = 0
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Association between Exposure, E, and Disease, D
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iff a/(a+b) ≠ c/(c+d) i.e., RR ≠ 1 OR ≠ 1 Risk difference ≠ 0
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Guidelines for judging whether an association is causal (Bradford Hill's criteria, modified)
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temporality, strength of the association, dose-response relationship (biologic gradient), replication of the findings (consistency), coherence with other knowledge, specificity of the association (weakest of all guidelines), cessation of exposure (experimentation), analogy, consideration of alternate explanations (Gordis)
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Hippocrates, 5th Century BC
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Disease related to both external and personal environment
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John Graunt, 1662
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Weekly reports of births & deaths; Quantified patterns of disease in a population; Seasonal variation in mortality
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William Farr, 1839
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In charge of medical statistics; elevation above sea level and deaths from cholera
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John Snow vs Henry Whitehead
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Disease spread, food and water contamination, miasma; on the mode of communication of cholera"
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Jenner
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cowpox immunization for smallpox
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Frost
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transmission of tuberculosis contribution to surveillance new cases
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Austin Bradford Hill
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Pioneered randomization component of RCTs
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Doll and Hill
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Pioneered case-control study; first to show link between smoking and lung cancer
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Clinical Epidemiology
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Application of epidemiologic methods to clinical research education, and patient care
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Deaths due to CVD
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11,940 811,940 deaths due to CVD ( 2,225/day); 32.8% of all deaths (~1 of every 3 deaths); Mentioned on 55% of all death certificates; If all forms of major CVD were eliminated, life expectancy would rise by almost 7 years(3 years for cancer)
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Confounding
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A situation in which the measure of association is distorted because of the relationship between the exposure and other factor(s) that also influence the outcome.;It is a true phenomenon, and not an error in the study.
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Selection Bias
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Error due to systematic differences in characteristics between those who take part in a study and those who do not; The problem is that the association between exposure and outcome may differ between those who participate in the study and those who do not; Usually inferred, rather than observed (Self-selection bias, selection of controls, healthy worker effect, post-entry exclusion bias)
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Information Bias
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A flaw in collecting or measuring exposure or outcome data that results in different quality/accuracy of information between comparison groups (Misclassification, differential and non-differential with respect to exposure and outcome status, recall bias, reporting bias, Interviewer bias
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Confounding
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A situation in which the measure of association is distorted because of the relationship between the exposure and other factor(s) that also influence the outcome; it is a true phenomenon, and not an error in the study
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Combined Total Risk of A and B (additive effect)
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Baseline Risk + Attributable Risk (A) +Attributable Risk (B)
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Combined Effect of A and B (additive effect)
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Attributable Risk (A) + Attributable Risk (B)
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Combined Total Risk of A and B (Multiplicative effect)
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Baseline Risk x Relative Risk (A) x Relative Risk (B)
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Combined Effect of A and B (Multiplicative effect)
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Relative Risk (A) x Relative Risk (B)
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Cancer
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Mass of cells characterized by acquired genetic changes heritable by progeny cells; Inappropriate, uncontrolled proliferation; 30% of all cancer deaths are due to smoking; 90% of all lung cancers deaths are due to smoking
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Ecologic
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study populations Measure of association: Correlation coefficient r + quick and cheap, hypothesis generation- difficult to interpret, ecologic fallacy, no OR
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Clinical trial
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controlled experiment to evaluate efficacy and safety of treatment Measure of association: RR: rate of treated/rate of untreated RR reduction: 1-RR + exposure, gold standard of comparative studies, protect against randomization bias - external validity of absolute effect of treatment for entire population
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Cross-sectional
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Info on exposures, outcomes, and other factors collected at same time Measure of association: Prevalence ratio (prev exposed/prev un) and diff (prev exposed - prev un) + generate hypothesis, basis for health policy - no temporality or causality
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Cohort
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start w/exposed and non, follow to see who develops Measure of association: when closed, RR (inc in exp/inc in non). When open, incident rate ratio + Prevalence and incidence possible, temporality - Long time, expensive, causality Closed: individuals enter at same time, exit if they have endpoint of interest or never have event
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Case-control
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compare case (have disease) and control (no disease) Measure of association: Odds of exposure (cases a/c, controls b/d) Odds ratio: AD/BC + OR approx. RR when cumulative incidence <10%, good for rare diseases (non-nest) - no prevalence or incidence, difficult to study rare exposure, no temporal sequence Nested: source pop is ongoing cohort study (requires passage of time)
