Mother and Child Health: Research Methods G.J.Ebrahim Editor Journal of Tropical Pediatrics, Oxford University Press.

Case-Control Studies Cases Controls Population at risk Exposure Yes No Yes No

Steps in Designing Case-Control Studies Selection of cases. Precise definition of ‘case’. Inclusion / Exclusion criteria. Are cases to be ‘incident’ or ‘prevalent’? How are cases to be identified? How recruited? Selection of Controls. Source ( hospital patients without disease; neighbourhood controls; random sample of population; sibs). Inclusion / exclusion criteria. Match to cases? Collection of information. Identify risk factor of interest. Method of collection of information ( questionnaire; medical records; employment records). Same procedure to be used for cases and controls. Interviewer should be unaware who is a case and who a control.

Two Methods of Selection Select new cases (i.e.incident) as they come up. Controls are selected from those in the same setting at the same time. Select existing cases (prevalent) from a defined population. From the same population a larger number of controls are identified. The Incident type of case-control study is stronger because diagnosis of cases and ascertainment of exposure is being done by the researcher.

Calculation of risk Exposed Not Exposed CasesNon-cases AB CD Odds Ratio = A×D ÷ B× C

Matched case - Control Studies cases Non-cases cases Non-cases All cases or random sample cases Random sample of non-cases cases All cases or random sample Controls Matched controls

Advantages and Disadvantages of case-control studies. Relatively cheap compared to cohort studies Relatively quick Useful for study of rare diseases. No ethical problems Useful for diseases with long latent period. Estimate of disease incidence cannot be done At times difficult to measure exposure accurately Open to selection bias. Difficult to interpret. Advantages Disadvantages

Points to be careful About Confounders. Groups are self allocated and may have other risk factors with different distribution Selection bias. Controls should be a representative sample of the same population where cases came from. Recall bias. We ask about exposure after the disease has been diagnosed. Interviewer bias. Best dealt with by “blinding” to hypothesis and whether subject is case or control. Also use same interviewer for cases and controls, and verify responses.