2.5 Study Analysis

2.5 Study Analysis

1. Cohort Studies: Binary Exposure

   • Consider the cohorts. They have been categorised according to their exposure status and also by their disease status.
     • The results of the study can then be illustrated using the following general tabular representation

2. Case-Control Studies: Binary Exposure

   • Sampling of cases and controls is based upon their disease status
     • Information regarding exposure is then obtained retrospectively
       • It is not possible to estimate disease risk, differences in disease risk or relative risks since sampling is undertaken based upon disease status
   • Consider the exposure status of the cases (diseased) and controls (non-diseased) have been ascertained
     • The results of the case-control study can then be illustrated using the following general tabular representation
   • Selecting Cases and Controls
     • The number of available cases is often limited
       • So the cases are often a complete enumeration of the diseased
         • i.e. all known disease cases in a region in a specified time period
     • Controls may be more readily available
       • Thus we can choose the number of controls
         • We can increase the precisions of our estimate for the disease odds ratio by increasing the number of controls
           • Beyond 5 controls per 1 case yields little benefit
       • Controls may be a simple random sample from the disease free population at risk
       • Controls may be matched to the cases
         • Matching is used to handle known confounders

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