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Epidemiology Methods

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Epidemiologic methods are the concepts, frameworks, tools, and techniques used in contemporaneous epidemiological research and practice

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Epidemiology

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The study of the occurrence and distribution of health-related states or events in specified populations, including the study of determinants influencing such states, and the application of this knowledge to control the health problems.

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Sanitary statistics era

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First half of 19th century; Miasma theory of disease; Analytic approach clusters of morbidity and mortality; prevention through sanitation

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Infectious disease era

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Late 19th century through first half of 20th century; germ theory of disease; analytic approach is laboratory isolation of disease; prevention through vaccination

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Chronic disease epidemiology era

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Later half of 20th century; black box approach, exposure related to outcome; analytic approach is risk ratio of exposure to outcome at individual level in population; goal to control lifestyle

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Eco-epidemiology era

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current era; Chinese box or relationships at different hierarchies and levels; analysis of factors at different levels

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Population

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Any sizable aggregate of people..who satisfy a particular set of membership criteria

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Membership criteria

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person, place & time

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challenges with personal charecteristics

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lack specific characteristics or hard to define; people may not want to acknowledge characteristics due to social constraints

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Symptoms

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Subjective indications reported by the individual

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Signs

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Objectives indications apparent to the observer

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Tests

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lab, physical, psych, etc.. indicate presence of disease

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Types of cases

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confirmed, probable or possible

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Confirmed cases

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Usually laboratory-confirmed

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Probable cases

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Clinical characteristics, no labs

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Possible cases

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Usually fewer of the clinical characteristics

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More encompassing, less stringent case definition

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typically fewer criteria and risk labeling people cases when they are not cases

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less encompassing, more stringent

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typically more criteria and risk labeling people noncases when they are (truly) cases

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at risk

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Able to experience and measure the outcome of interest

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more encompassing, less stringent

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typically identifies less true cases but people who are non cases will more likely be correctly identified as non-cases

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susceptibility

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not immune to an infectious agent because of prior infection to the agent, vaccination, maternal antibodies (infant)

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biological considerations for "at risk" population

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susceptible to disease and have target organ

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censoring

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person did not experience the outcome of interest and is no longer under follow up

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administrative censoring

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person did not experience the outcome of interest and is no longer under follow up because study has ended

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closed population

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everyone enters population at same time, and only exits for the outcome of interest, and there is no administrative censoring

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epidemiological curve

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Graphical depiction of the distribution of the times of onset of a disease in the population

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Incubation period

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Interval from origin to time of onset of clinical illness

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Attack Rate/ Cumulative Incidence

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Proportion of a study population who become cases and develop the disease of interest by a certain time

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Attack rate equation

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Number of who get disease during time t/ Total number of people at risk during time t (not a rate, actually a proportion)

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Epidemic

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The occurrence in a community or region of cases or illness clearly in excess of normal populations

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Disease outbreak

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An epidemic limited to localized increase in the incidence of a disease, e.g., in a village, town, or closed institution;

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Endemic disease

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The constant presence of a disease within a given geographic area or population group; also the usual frequency of a given disease within such an area or group

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Point source epidemic

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Persons are exposed to the same exposure over a limited, defined period of time, usually within one incubation period

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Point source epidemic curve

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The shape of epidemic curve commonly rises rapidly and contains a definite, followed by a gradual decline

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Point source with secondary transmission curve

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cases may appear as a wave that follows a point source by one incubation period or time interval

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Propagated epidemics

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Case of disease serves as a source of infection for subsequent cases and those subsequent cases, in turn, serve as sources for later cases.

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Propagated epidemic curve

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The shape of the curve usually contains a series of successively larger peaks, reflective of the increasing number of cases caused by person-to- person contact, until the pool of susceptibles is exhausted or control measures are implemented.

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Risk ratio

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Reflects the rela1ve difference of the occurrence of disease frequency in one group vs. another

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Risk difference

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Reflects the absolute difference of the occurrence of disease frequency in one group vs. another

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Natural history of disease

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reflects the sequence and progression of disease pathogenesis

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Loss to follow up

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censoring when a person leaves the study early

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competing risks

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censoring when a person has another event that puts them no longer at risk for the outcome of interest

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Late entries/ staggered entries/ left censoring

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entry of individuals who fulfill membership criteria but were not "at risk" at the start of follow up

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Open population

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gain members over time, through immigration or birth, or lose members who are still alive (or disease free) through other reasons

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Time scale or time axis

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Yardstick by which time is measured

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person time

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amount of time a person was followed in the study; used in open populations

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person time assumption

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assume risk of the outcome in any given moment of follow-up contributed by any given individual is similar

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target population

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the group of individuals to whom inferences are to be made

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source population

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group of individuals expected to have the same measure of disease or association as the target population and who can be enumerated

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study population

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group of individuals under observation in the study, who are expected to have the same measure of disease or association as the source population

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analytic population

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subset of the study population that is eligible for a particular analysis

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Randomized clinical trial

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Subjects in a population are randomly allocated into groups, usually called study and control groups, to receive or not to receive an experimental preventive or therapeutic procedure, maneuver, or intervention ('treatment').

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concealment of random allocation

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Keeps study investigators and staff and potential participants unaware of upcoming assignments; occurs before study begins

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masking or blinding

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Keeps study investigators and staff and participants unaware of their assignment; occurs after study begins

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factorial design

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a type of randomized study where an individual randomly assigned to one treatment and then another second treatment; can study multiplicative effects or two unrelated treatments

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cohort study

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subsets of a defined population are followed over time depending on exposure and studied for outcome

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cohort

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common experience and/or grouping; eg. age, risk or clinic based

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prospective cohort study

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the protocol and cohort are established before people are followed - i.e., all events occur after the cohort is established

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retrospective cohort study

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the protocol and cohort are assembled after events have already occurred. Follow-up of cohort starts in the past and events are determined from past information

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case/ control study

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observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease.

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suitable controls in case control study

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individuals who could have had the disease of interest but didn't; must be from the same source/ target population

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nested case control study

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a study that occurs within a cohort study; controls are selected from the cohort when cases occur

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nested case cohort study

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controls are selected from cohort at beginning

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nested case crossover study

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Cases serve as their own controls

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cross sectional study

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Study that examines the relationship between diseases (or other health-related characteristics) and other variables of interest as they exist in a defined population at one particular time

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ecological study

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A study in which the units of analysis are populations or groups of people rather than individuals

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aggregate measure

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summaries of the distribution of an individual-level measure

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intrinsic measure

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measures that apply to populations and cannot be measured on an individual level

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reasons to conduct an ecological study

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measurement limitations or design limitations

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measurement limitation

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in environmental epidemiology often cannot measure relevant individual exposures

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design limitation that necessitate ecological study

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individual exposure may vary minimally in one geographic area, and uniformity in exposure does not help to make inferences

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direct exposure

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actual agent that may be causing disease; hard to measure so instead use surrogates to approximate the level of exposure

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surrogate exposure

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not the exposure directly cuasing disease but is either associated, on the causal pathway, or a biomarker; used to approximate direct exposure, and easier to measure than direct exposure

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individual level exposure

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measured for individual persons in population

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group level exposures

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aggregate attributions of individuals, physical characteristics of location, describing interactions of individuals, cannot be reduced to individual level

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administered dose

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amount of exposure coming in contact with individual in defined time period, intake

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internal dose

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amount of agent exposed or deposited inside the body in a defined time period, uptake

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biologically effective dose

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amount of agent that actually interacts with target sites in the body

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fixed exposures

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don't change over follow-up or with additional measurement

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time varying exposures

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exposures that are time-dependent, modifiable, and may change over time

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components of summarizing exposure

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duration, intensity and frequency, examples include average exposure, cumulative exposure, peak exposure

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allocating person time

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person time can be binary (exposed or unexposed) or can be time dependent; allows a person to transition through categories of exposure

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direct outcome measurement

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measure the outcome that occurs, when it occurs

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surrogate outcome measurement

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measure a medial point between true outcome, use this outcome as a surrogate for the direct outcome; take less time to develop and may be more common, but may be less clinically meaningful

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clinical outcomes

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easily interpretable results, applicable to clinical practice but late in the disease process and can be costly

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subclinical outcome

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used as a surrogate for clinical disease, earlier in disease course but less clinically meaningful

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classifying cases by level of certainty

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confirmed cases, probable cases, possible cases

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active ascertainment

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An approach where the investigator initiates (or attempts) contact with participants to determine if the outcome occurred.

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passive ascertainment

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An approach when the investigator uses indirect tracking of status of participants.

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consequence of imperfect measurement in continuous variable

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measurement error

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consequence of imperfect measurement in discrete varaible

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misclassification error

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misclassification error

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Erroneous classification of individual, a value, or an attribute into a category other than that to which it should be assigned. Due to random error and bias

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Observer/diagnostic error

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after data classification, error in person conducting test

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Instrument/ method error

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error in instrument used, or method in study

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reporting or transmission error

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correct result is obtained but error in reading the measurement

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recording error

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correct result is obtained but it is recorded incorrectly

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entry error

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the correct result is entered incorrectly

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measured value equation

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measured value = true value + error

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error equation

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error= random error + bias

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bias

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systematic component of error; difference between true value and measured value

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random error

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random component of error

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replicates

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repeated measurements to assess error

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extent of random error

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variability, spread or dispersion

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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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binary variable

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can calculate sensitivity and specificity to compare test to gold standard

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sensitivity

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Proportion correctly classified as having the attribute ("case") by the measure under study ("new test") compared with the "gold standard" measure

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specificity

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Proportion correctly identified as NOT having the attribute ("non-case") by the test measure compared with the gold standard measure

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negative predictive value

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The probability a negative test result is a true negative.

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validity definition

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An expression of the degree to which a measurement measures what it purports to measure.

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correlation coefficient (r)

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"Measure of association that indicates the degree to which two variables have a linear relationship; spearman's and pearson's

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Pearson's correlation

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Pearson's correlation measures how close the data is to the "line of best fit" NOT fit to the "line of agreement"

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regression

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slope of a line to determine whether there is bias; slope of less than one is overestimate, and slope greater than one is an underestimate

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validity of data with binary distribution

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sensitivity and specificity

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validity of ordinal discrete data

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Spearman's rank correlation of coefficient

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validity of binary or ordinal data

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percent agreement or kappa statistic

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measurement options for reliability

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percent agreement, percent positive agreement, kappa

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percent agreement

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The proportion of individuals who are classified the same by both tests

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