Analysis of Clinical Trial Data

The analysis of clinical trial data is very similar to the previously described analysis of data from a cohort study. The first step is to generate simple descriptive statistics on each of the groups being compared in order to characterize the study population and alert you and your readers to any differences between the groups with respect to other exposures that might cause confounding. If large numbers of subjects have been randomly assigned to the treatment arms, the groups should be comparable. If there are more than minor discrepancies, the investigators need to review the randomization procedures and consider adjusting for confounding by other methods.

The table below shows just a portion of the data from the table of descriptive statistics from the Physicians' Health Study on aspirin.

  Aspirin (n=11,037) Placebo (n=11,034)
Age (years) 53.2 ± 9.5 53.2 ± 9.5
Systolic BP (mm Hg 126.1 ± 11.3 126.1 ± 11.1
Diastolic BP (mm Hg) 78.8 ± 7.4 78.8 ± 7.4
History of hypertension (%) 13.5 13.6
History of high cholesterol (%)


17.5 17.3
Cholesterol level 212.1 ± 44.2 212.0 ± 45.1
History of diabetes (%) 2.3 2.2

Note that the two groups were remarkably similar on these and other characteristics, indicating that randomization had been successful.

After generating the descriptive statistics, the next step is to generate crude estimates for the magnitude of association between the primary exposure and the outcomes of interest.

Test Yourself

After 5 years of follow up In the Physicians' Health Study, an interim analysis found that among the 11,034 men assigned to the placebo group there had been 213 non-fatal myocardial infarctions. Among the 11,037 men assigned take 325 mg. of aspirin every other day, there had been 126 non-fatal myocardial infarctions.

Summarize these finding in a contingency table and compute the cumulative incidence in each group, the risk ratio, and the risk difference. Then interpret the risk ratio and the risk difference. Complete all of these tasks before comparing your answers to the ones provided in the link below.