BC Jung A Brief Introduction to Epidemiology - IX (Epidemiologic Research Designs: Case-Control Studies) Betty C. Jung, RN, MPH, CHES

BC Jung Learning/Performance Objectives u To develop an understanding of: –What case-control studies are –The value of such studies –The basic methodology – Pros and Cons of such studies

BC Jung Introduction Epidemiology studies the distribution of disease in a number of ways. The two major categories of epidemiological studies are: Observational and experimental studies. Most epidemiological studies are observational.

BC Jung Epidemiological Study Designs u Observational Studies - examine associations between risk factors and outcomes (Analytical - determinants and risk of disease, and descriptive - patterns and frequency of disease) u Intervention Studies - explore the association between interventions and outcomes. (Experimental studies or clinical trials)

BC Jung Research Designs in Analytic Epidemiology u Ecologic Designs Cross- Sectional Study u Case-Control Study u Cohort Study

BC Jung Case-Control Studies u “Flashback Studies” (Paffenbarger, 1988) u Retrospective - compare cases and controls for presence of disease u Includes passage of time. u Historical - assess past characteristics or exposures in two groups of people- cases and controls

BC Jung Examples u The relationship between thalidomide and unusual limb defects in Germany u The relationship between meat consumption and enteritis necroticans in Papua New Guinea

BC Jung Value u Simple to conduct u Cost-effective way to study a rare disease

BC Jung Case-Control Studies: Methodology Then measure, post-exposure First Select the Cases and Controls Cases (with disease) Control (without disease) Were Exposed AB Not Exposed C D A+CB+D Population Exposed A/A+CB./B+D

BC Jung Case Control Design Time Direction of Inquiry Population Cases with the Disease Controls without the disease Exposed Not Exposed

BC Jung Case-Control Studies u Cases - Has condition or health outcome of interest. Has higher frequency or greater degree of exposure than non-cases. u Controls (non-cases) - Does not have the health condition. Serves as the comparison group u Ask about history of contact with or exposure to supposed causes

BC Jung Case-Control Studies u If controls are well chosen, the only antecedent difference will be in the level of a characteristic that is related causally to the development of a disease (I.e, exposure to a chemical resulted in cancer). u Quantify with odds ratios

BC Jung Strength of Association Relative Risk;(Prevalence); Odds Ratio Strength of Association None Weak Moderate Strong 10.0 Approaching Infinity

BC Jung Methodology Issue: Matching u Matching - control for confounding variables. If you do not match then control by subject selection (study only males to eliminate gender as a confounding variable) u Matching –Subject selection –Statistical control during data analysis –The more variables that need to be matched the greater the universe we need. –Problem - match age, sex and SES - Control must be the same.

BC Jung Methodological Weaknesses u Biased reporting of the antecedent (having lung CA -> patients over reporting smoking (from guilt, knowledge or selective memory) u Subject selection (decreases with cases and controls in the same facility) u Limited to only cases, who have survived at the same time. Selective survival

BC Jung Pros u Cases easily available u Good for less common or rare cases u Quick, inexpensive u Can be conducted by clinicians in clinical facilities u Tend to support, not prove causal hypothesis by establishing associations u Historical data available in clinical records u Number of subjects needed is small

BC Jung Cons u Info about antecedents depends on memory, which could lead to bias u Clinical data may be inadequate or incomplete u “Case group” may not be homogenous - criteria for diagnosis may differ. u Clinical cases are selective survivors

BC Jung Cons u Non-representativeness of cases. Those coming in for treatment may differ from those not seeking treatment and those going somewhere else. u Antecedent is not obtained from universe of all antecedents. u Berkson’s fallacy - making generalizations from hospital or clinical samples to the general population. u Cannot know what association would be for all or for a representative sample of all people having the antecedent.

BC Jung References u For Internet Resources on the topics covered in this lecture, check out my Web site: u