Components of variance have a long history and find application in all areas of scientific investigation. This review introduces components of variance and their importance firstly by examples on blood pressure, proteomic data, breath analysers and esophageal pH monitoring devices. We then present an intuitive geometric representation of analysis of variance and explain how the components of variance can be estimated from the analysis of variance table. We conclude by suggesting practice points for studies which incorporate components of variance, and recommend commonly used statistical software to undertake such analysis. Refereed/Peer-reviewed