Understanding statistical analysis in the surgical literature: some key concepts

Understanding the fundamentals of statistical analysis allows surgeons to evaluate the logic and validity of statistical methods presented in published papers in order to make a judgement about the quality of the reported findings. This paper presents an overview of some basic concepts, with an emphasis on principles rather than on mathematical detail, which relate to selecting the correct statistical test and interpreting the results of statistical analysis. The most important factor in determining the appropriate significance test is the type of outcome data that have been collected. A significance test is used to calculate a probability (P) value, which is a measure of the strength of the effect seen in the study, but additional useful information is obtained from confidence intervals that indicate not only the size and direction of the effect but also the precision of the study. Judgements about the clinical importance of a result should be based on the size of the effect seen rather than the P value, as the latter is strongly influenced by the size of the study.