IV. Appropriateness of the Statistical Test

Abstract

When we observe the difference between two therapies or the association of a risk factor or prognostic indicator with its outcome, we have to assess the certainty of the result. This assessment is based on a judgement that uses information related with the design of the study and the statistical handling of the information. In this article, the relevance of the selected statistical test is specifically mentioned. Statistical tests are chosen based on two features: the objective of the study and the type of variables. The objective can be divided in three groups of tests: a) those in which showing differences between groups or in a same group before and after a maneuver is wanted; b) those in which showing a relationship between variables is wanted; c) those in which predicting an outcome is pretended. As for the types of variables, we have two: quantitative (continuous and discontinuous) and qualitative (ordinal and dichotomous). For example, if we want to demonstrate age differences (quantitative variable) between patients with systemic lupus erythematosus, with and without neurological involvement (two groups), the adequate test is Student’s t-test for independent samples; but if what is being compared in those same groups is the frequency of females (binomial variable), then the relevant statistical test is the chi-square test (c2).