The following information is needed before the calculation can be done:
• required level of statistical significance of the ability to detect a difference
• acceptable error, or chance of missing a real effect
• magnitude of the effect under investigation
• amount of disease in the population
• relative sizes of the groups being compared. In reality, sample size is often determined by logistic and financial considerations, and a compromise always has to be made between sample size and costs. A practical guide to determining sample size in health studies has been published by WHO.
The precision of a study can also be improved by ensuring that the groups are of appropriate relative size. This is often an issue of concern in case-control studies when a decision is required on the number of controls to be chosen for each case. It is not possible to be definitive about the ideal ratio of controls to cases, since this depends on the relative costs of accumulating cases and controls. If cases are scarce and controls plentiful, it is appropriate to increase the ratio of controls to cases. For example, in the case-controlstudy of the effects of thalidomide , 46 affected children were compared with 300 normal children. In general, however, there may be little point in having more than four controls for each case. It is important to ensure that there is sufficient similarity between cases and controls when the data are to be analysed by, for example, age group or social class; if most cases and only a few controls were in the older age groups, the study would not be able to account for the confounding factor of age.