The term “interim analysis” can, in its broadest sense, be used in relation to any evaluation of data undertaken during an ongoing trial. Whether examination of the data presents ethical and analytical challenges depends on the purpose of the analysis.
Some routine monitoring of trial progress, usually blinded to treatment allocation, is often undertaken as part of a phase III trial. This monitoring can range from simple checking of protocol compliance and the accurate completion of record forms to monitoring adverse events in trials of serious conditions so that prompt action can be taken. Such monitoring may be undertaken in conjunction with a data and safety monitoring board (DSMB), established to review the information collected. As no direct comparison of the treatments in terms of their benefit is undertaken at such interim analyses, special methodology is not required.
A second purpose of interim analyses is to undertake a sample size review, when the purpose of the interim analysis is to estimate one or more nuisance parameters (for example, σ2 in the case of normally distributed data), and this information is used to determine the sample size required for the remainder of the trial. Sample size re-estimation based on the estimation of nuisance parameters, particularly on the variance of normally distributed observations, was proposed by Gould and Shih . A review of the methodology is given by Gould . A sample ...