Epidemiology 4 – Study Types – Flashcards
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What is the purpose of descriptive epidemiological studies?
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Designed to describe the distribution of health-related events by person, place, time.
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T/F: Epidemiological studies always measure causal associations
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False - they suggest the existence of an association
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Profile studies of cases with similar symptoms of epidemiological interest
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Case report Case series
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Measures disease prevalence over a short period of time
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Cross-sectional or Prevalence study
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Use aggregated data at a population level to describe the relationship between two variables
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Ecologic study
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Association between meat consumption and colon cancer - what type of study is this?
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More meat consumption = higher risk for colon cancer. This is an example of the ecologic study
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The statement that an individual's "risk" can be determined by an epidemiologic study
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Epidemiologic fallacy. Because epidemiology investigates group-level data and draws conclusions on that level only.
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A study designed to examine a real or hypothesized causal association
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Analytical studies -Experimental -Observational
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This type of study has conditions that are under the direct control of the investigator.
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Experimental study
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The unit of treatment allocation in a study is an entire community or political subdivision
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Community trial Example: water fluoridation community trial
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The unit of treatment in a study is an individual; this type of study is smaller scale
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Clinical trial
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What are two main issues with clinical trials?
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1. Eliminating bias 2. Ethical considerations
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What are 5 ways to eliminate bias?
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1. Protocol preparation 2. Selection 3. Randomization 4. Blinding 5. Compliance
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An epidemiological study in which the investigator does not intervene in the treatments or exposures, but only notes the effects of "nature"
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Observational epidemiological study
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What is the difference between a follow-up (cohort) study and a case-control study?
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- Follow up study: exposure is known, disease endpoints to be determined - Case-control: disease is known, exposures to be determined (working backwards)
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Benefits of descriptive epidemiology
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1. Distribution of disease in population subgroups 2. Guides healthcare planning 3. Discover emergence of patterns
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After looking at the patterns from descriptive studies, what can be done?
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1. Analytical studies - associations among variables and risk (Case/control and follow up studies). 2. Formation of hypotheses
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From the hypothesis formed from an analytical study, what can be done?
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Experimental study - testing of interventions (clinical trials). Helps build the standards of medical practice
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Studying cohorts of persons forward in time; the subject is recruited after the exposure of interest but before disease occurs.
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Prospective study
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Studying cohorts of persons looking backward in time; the subject is selected for study after the disease occurred, but has a documented exposure occurring prior to disease
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Retrospective study
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Practical benefit of retrospective studies
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It is a common method for occupational epidemiological study or studies using mortality data
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The parameter generated from a cohort or follow up study
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Relative risk Rate ratio
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What is the "gold standard" of epidemiological studies?
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Cohort or follow up studies
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Examples from class of cohort studies
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1. Framingham Heart Study 2. Nurses Health Study 3. Black Women's Health Study
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A cohort reconstructed with data on the predictor variable (measured in the past) and data on the outcome collected (measured in the past after some follow up period)
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Historical cohort, retrospective study
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Specific aims of exposure reconstruction
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1. Identify materials used, tasks and occupations associated with production processes. 2. Estimate the expected range of exposures for tasks and occupations 3. Verify modeled estimates 4. Develop cumulative exposure estimates for job titles
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The parameter generated by a case/control study
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Odds ratio
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Steps in case control study
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1. Identify cases (persons experiencing a health-related state or event) 2. Identify controls 3. Investigate whether the cases are more or less likely than controls to have had past experiences, lifestyle behaviors, or exposures
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First step in conducting a case-control study? (Specifics)
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Establish diagnostic criteria and definition of the disease
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Differentiate between incidence & prevalence
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Incidence - NEW cases during a specific time period in specified population and location. Prevalence - EXISTING cases at a point in time
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What are the sources of cases for studies? (5)
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1. Records from public health clinics 2. Physician offices 3. Health maintenance organizations 4. Hospitals 5. Industrial and government sources - disease registries
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What is the goal when selecting cases?
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Cases should be representative of all persons with the disease
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What is the goal of control subjects?
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Control subjects should be representative of the general population, and look like the case subjects with the exception of not having disease - i.e. they have the same possibility of being selected or exposed as the cases
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Where are control subjects selected from?
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1. General populations 2. Hospital 3. Family, friends, relatives
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Where might information about exposure status be obtained from?
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1. Medical records 2. Interviews 3. Questionnaires 4. Surrogates (spouses, siblings, employers)
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When will an Odd Ratio be approx. equal to the relative risk?
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When the cases in the study are representative of all the cases. When the control in the study are representative of the general population. The frequency of the disease in the population is small.
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What do we mean when we say there is a "relative risk of 5?"
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The risk is 5 times greater when exposed to that particular risk factor. Attributable risk - what is the impact of the risk factor on that population?
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Calculate the odds ratio
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Crossproduct of 2x2 square: (a x d) / (b x c)
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Advantages of a cohort follow-up study
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1. Useful for studying rare *exposures* 2. Generates relative risk 3. Can generate incidence rates
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Disadvantages of a cohort follow-up study
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1. Normally requires a large sample size 2. Requires long follow up periods 3. Costly
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Advantages of a case/control study
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1. Useful for studying rare *diseases* 2. Quick results 3. Smaller sample required 4. Less costly
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Disadvantages of a case/control study
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1. Recall bias 2. Control selection may be difficult
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A systematic error in the collection or interpretation of data, resulting in over- or under-estimation of the association between exposure and disease
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Bias
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Where in the study should bias be avoided? Why?
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Avoid it at the design stage because of the difficulty in identifying and accounting for it thereafter
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Types of bias in case control studies
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Selection Observation - recall (of case vs. control) or interviewer (probing cases vs. controls differently)
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Occurs when an extrinsic factor is associated with a disease outcome and, independent of that association is also associated with the exposure
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Confounding
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A strategy for controlling confounding at both the design and analysis levels of a study
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Matching
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General principles of a study design (8)
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1. Define hypothesis 2. Define parameters of interest 3. Design study - f/u, case/control, ecologic 4. Define population - sampling, controls, study group 5. Collect data 6. Analyze data 7. Interpret results 8. Report findings
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Which things should be considered in the study design?
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1. Is it informative? Valid? Generalizable? 2. Is the research significant? To science? Public health? 3. Is the research feasible? Cost, time, efficiency, ethics?
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Comparison of two things
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Ratio
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Proportion related to time
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Rate - person years as denominator, uses a constant multiplier
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Number of existing cases of disease / population of interest
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Prevalence Reported as a decimal Per 100,000
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Number of new cases of disease / population of interest in person time of observation
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Incidence
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Correlation between exposure and outcome
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Association
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Variables change at the same time
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Correlation
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When an association is thought to be causal it is called __________
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A risk factor
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The amount of a factor that an individual or group has experienced
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Exposures
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Examples of types of exposures
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1. Contact with or proximity to an object or substance (virus, toxin, radiation, etc.) 2. Practicing a behavior (smoking, exercising, etc) 3. Possessing a characteristic (race, age)
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# existing cases in a year / total population at risk
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Prevalence
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# cases of illness during an outbreak
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Attack rate
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The "gold standard" measurement in epidemiology
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Relative risk
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Relative risk of two groups - calculation
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Prevalence or incidence group A / prevalence or incidence group B
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Calculation of relative risk
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Incidence in the exposed / incidence in the non-exposed
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- "Disease burden" - "Disproportionately infected or affected" - "Observation greater than expectation"
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Excess risk
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The rate or incidence of disease in an exposed group that can be attributed to the exposure. Used to determine the amount of disease that could be eliminated by removing the exposure.
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Attributable risk
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Calculate: attributable risk
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Prevalence or incidence (exposed) - prevalence or incidence (not exposed)
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Calculate: population attributable risk (PAR)
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Prevalence or incidence (population) - Prevalence or incidence (not exposed)
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Calculate: attributable risk percent in exposed
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AR / incidence or prevalence of exposed
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Calculate: attributable risk percent in population
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PAR / incidence or prevalence in the population
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Measures the strength of an association
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Relative risk
