Study of Design and ethesocs and biostatistics and research methodology

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1. Introduction The goal of a case-control study is the same as that of cohort studies, i.e., to estimate the magnitude of association between an exposure and an outcome. What is the probability of having been exposed if you have the disease compared to not having the disease? However, case-control studies employ a different sampling strategy that gives them greater efficiency.

2. Case-Control Study Design Two groups of individuals are compared, one who have been diagnosed as diseased with those who are free from disease. The group having disease is called Cases while the disease–free group is called Controls. In a case-control study, if there is an association of an exposure with a disease, the prevalence of history of exposure should be higher in persons who have the disease (cases) than in those who do not (controls).

3. Designing a Case-Control Study

4. Case-Control Study

5. Case-Control Study

6. Case-Control Study If exposure is associated with disease, we would expect the proportion of the cases who were exposed, or a/ a+c , to be greater than the proportion of the controls who were exposed, or b/ b+d .

7. Case-Control Study: Example of CHD and Smoking

8. Case-Control Study: Example This calculation is only a first step. Further calculations to determine whether or not there is an association of the exposure with the disease will be discussed later

9. Basics The hallmark of the case-control study is that it begins with people with the disease (cases) and compares them to people without the disease (controls). What about Cohort Study? It begins with a group of exposed people and compares them to a non exposed group.

10. Continue.. There is wrong impression that the distinction between the two types of study design is that cohort studies go forward in time and case-control studies go backward in time. A retrospective cohort study also uses data obtained in the past. Therefore, calendar time is not the characteristic that distinguishes a case-control from a cohort study.

11. Designing a Case-Control Study

12. Example: Case-Control Study of Lung Cancer and Tobacco Smoking There are fewer heavy smokers among the controls and very few nonsmokers among the lung cancer cases , a finding strongly suggestive of an association between smoking and lung cancer. Taken into account the dose of the exposure is very important.

13. Continue.. When an association is documented in a case-control study, the next step is often to carry out a cohort study to further explain the relationship…….why not to start with cohort study? Because case-control studies are generally less expensive than cohort studies and can be carried out more quickly . They are often the first step in determining whether an exposure is linked to an increased risk of disease.

14. Continue.. Case-control studies are also valuable when the disease being investigated is rare . It is often possible to identify cases for study from disease registries, hospital records, or other sources.

15. POTENTIAL BIASES IN CASE-CONTROL STUDIES 1. Selection Bias: 2. Information Bias Problems of Recall A major problem in case-control studies is that of recall. Recall problems are of two types: a) limitations in recall and b) recall bias. Recall bias is the main form of information bias in case-control studies.

16. 2. Information Bias Problems of Recall Limitations in Recall . Much of the information relating to exposure in case-control studies often involves collecting data from subjects by interviews. Because virtually all human beings are limited to varying degrees in their ability to recall information, limitations in recall are an important issue in such studies. A related issue that is somewhat different from limitations in recall is that persons being interviewed may simply not have the information being requested.

17. 2. Information Bias Problems of Recall b. Recall Bias. A more serious potential problem in case-control studies is that of recall bias. Suppose that we are studying the possible relationship of congenital malformations to prenatal infections. We conduct a case-control study and interview mothers of children with congenital malformations (cases) and mothers of children without malformations (controls). Each mother is questioned about infections she may have had during the pregnancy.

18. 2. Information Bias Problems of Recall A mother who has had a child with a birth defect often tries to identify some unusual event that occurred during her pregnancy with that child. She wants to know whether the abnormality was caused by something she did. Why did it happen? Such a mother may even recall an event, such as a mild respiratory infection, that a mother of a child without a birth defect may not even notice or may have forgotten entirely. This type of bias is known as recall bias ; Ernst Wynder , a well-known epidemiologist, also called it “ rumination bias .

19. Basic Steps in Conducting Case Control Study Four basic steps Selection of Cases and Controls Matching Measurement of Exposure and Analysis and Interpretation