Methods and Applications of Statistics in Clinical Trials, Volume 1: Concepts, Principles, Trials, and Designs

Chapter 25

Covariates

Willi Sauerbrei

25.1 Universal Character of Covariates

In clinical trials, covariates are used for many purposes. They are often called explanatory variables, predictor variables, independent variables, prognostic variables, or prognostic factors. Covariates can be used in all phases of clinical studies, including defining the patient population, calculating the required sample size, allocating treatment in randomized and nonrandomized trials, adjusting the estimate of a treatment effect, defining subgroups considered for investigations of interactions with treatment, helping in the biological understanding of a disease and its potential treatment, and developing prediction rules. Covariates can be demographic factors such as country or race, or biological and genetic factors such as sex or blood group, or factors that may change over time such as blood pressure or tumor size, or character

Statistics of patients that are relevant for the course of the disease and the effect of a treatment. Covariates are measured on different scales, such as binary (sex), categorical (histologic tumor type), ordered categorical (tumor grade), or continuous (age, height). Because of their universality and general applicability, covariates play a central role in clinical research, in randomized controlled trials (RCT), and in non-randomized trials (NRT). Beyond this, covariates are used in medical decision making and health policy decisions. Statistics of patients that are relevant ...