Heparan Sulfate: A Complex Polymer Charged with Biological Activity

Several authors have addressed the problem of calculating sample size for a matched case–control study with a dichotomous exposure. The approach of Parker and Bregman (1986, Biometrics42, 919–926) is, in our view, one of the most satisfactory, since it requires specification of quantities that are often easily available to the investigator. However, its recommended implementation involves a computational approximation. We show here that the approximation performs poorly in extreme situations and can be easily replaced with a more exact calculation.