Case control study

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1. Case-Control Study

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3. Overview Types of Study Design Hierarchy of Evidence Application of all study design Introduction to Case control study Features of CCS Steps in CCS Potential Bias in CC Advantages and Disadvantages of CCS 3 Revision

4. Major Study Design 4

5. Hierarchy of Evidence 5

6. Application of different study designs 6

7. Introduction Aka Case-Referent Study Case Control study is the observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease (comparison group, reference group ) It is not correct to call “case-control study” any comparison of a group of people having a specific outcome with another group free of that outcome. 7

8. By definition, CC study have two population 1 2 8

9. Introduction Cases refer to people who have developed the disease or outcome in question. Controls refer to people who do not have the disease or outcome in question, and who are selected to estimate the frequency of the exposure in the population . 9

10. Why Case control study is often called as Retrospective Study ???? Case-control study goes from disease onset backwards to the postulated causal factors. 10

11. Features of Case control Study Both exposure and outcome (disease) have occurred before the start of the study The study proceeds backwards from effect (disease) to cause (exposure) It uses a control or comparison group to support or refute an inference. 11

12. Case-Control Vs Cohort 12

13. Nested Case control Study Design An important type of case-control study in which cases and controls are drawn from the population in a fully enumerated cohort . Typically , some data on some variables are already available about both cases and controls; thus concerns about differential (biased) misclassification of these variables can be reduced Example: environmental or nutritional exposures may be analyzed in blood from cases and controls collected and stored years before disease onset. A set of controls is selected from subjects (i.e., non-cases ) at risk of developing the outcome of interest at the time of occurrence of each case that arises in the cohort 13

14. When is a Case-Control Study Desirable ? When the disease or outcome being studied is rare . When exposure data is difficult or expensi ve to obtain. When the study population is dynamic . When little is known about the risk factors for the disease, case-control studies provide a way of testing associations with multiple potential risk factor 14

15. Steps in Case Control Study Selection of Cases and control Matching Measurement of Exposure Analysis and Interpretation 15

16. Steps in Case Control Study Selection of Cases : B ased on Case Definition : A set of criteria that must be fulfilled in order to identify a person as representing a case of a particular disease. Involves two specification: Diagnostic Criteria: Eligibility Criteria: Incident cases are eligible rather than Prevalent cases Sources of Cases Hospital General Population 16

17. Steps in Case Control Study Selection of Contro l—crucial process Features Free from disease Must be as similar to the cases as possible, except for the absence of the disease under study As a rule, a comparison group is identified before a study is done, comprising of persons who have not been exposed to the disease or some other factor whose influence is being studied . Difficulties… if disease exist in subclinical forms where diagnosis isn’t possible 17

18. Steps in Case Control Study Sources of Control: Hospital control Relatives Neighborhood control General Population How many controls are needed? If cases are easily available, case: control is 1:1 If cases are rare and less in number, case: control will be 1: 2 or3 or 4 18

19. Steps in Case Control Study Matching : Defined as the process of selecting the controls so that they are similar to the cases in certain characteristics, such as age, race, sex, socioeconomic status, and occupation Types: a) Group Matching b) Individual matching Why need matching??? If more cases than controls are found to have been exposed, we may be left with the question of whether the observed association could be due to differences between the cases and controls in factors other than the exposure being studied. 19

20. Steps in Case Control Study Group matching (or frequency matching ) consists of selecting the controls in such a manner that the proportion of controls with a certain characteristic is identical to the proportion of cases with the same characteristic . Example: If 25% of the cases are married, the controls will be selected so that 25% of that group is also married. 20

21. Steps in Case Control Study Individual Matching: A second type of matching is individual matching (or matched pairs ). In this approach, for each case selected for the study, a control is selected who is similar to the case in terms of the specific variable or variables of concern. For example, if the first case enrolled in our study is a 45-year-old white woman, we will seek a 45-year-old white female control. 21

22. Steps in Case Control Study Measurement of Exposure Questionnaire Interviews Lab. Other instrumental investigations Studying past record e.g hospital records. 22

23. Steps in Case Control Study Analysis To find out Exposure rate among case & control to suspected factor e.g. rates among exposed & unexposed Estimation of disease risk associated with exposure(odds ratio) 23

24. Analysis Exposure rate among case=50/61 Exposure rate among control=16/41 Odds ratio= is the ratio of the odds of exposure among the cases to the odds of exposure among the controls 24

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27. Potential Bias in Case-Control Study Control Selection Bias Case Selection Bias Neyman Bias Berksonian Bias Self selection Bias Information Bias: Observer Bias Recall Bias Confounding 27

28. Advantages of case control study Easy Inexpensive Few subjects Suitable for rare disease No risk to subject Study of several different etiological factor Risk factor can be identified No follow up needed Minimal ethical problem 28

29. Disadvantages of case control study Problem of bias Difficulty in selecting control group Estimate relative risk only. Representativeness of cases & control may be of concerned. 29

30. References k. park-park's textbook of preventive and social medicine 23rd Edition Leon Gordis , 5 th Edition Miquel Porta-Dictionary-of-epidemiology, 5 th Edition http:// sphweb.bumc.bu.edu/otlt/MPH-Modules/EP/EP713_Case-Control/EP713_Case-Control_print.html 30

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