When running experiments that involve humans or animals, for example in clinical or pre-clinical research, it is often the case that multiple measurements are taken on the experimental subjects. This may involve measuring subjects repeatedly over time to track any trends, administering a sequence of treatments to compare their effects within-subject, or taking multiple technical replicate samples of each subject to obtain a more reliable average response. However, it appears that there is some confusion as to what constitutes a 'repeated measure' and what does not. This paper clarifies this confusion by defining characteristics a design may possess when the subjects are assessed multiple times. These characteristics are delineated based on whether the subjects are the experimental units, if the experimental units are measured repeatedly or not, and which randomisation strategy is used. Experimental designs can then be classified using these characteristics and visualised using Hasse diagrams.

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