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Fig. 2 | Trials

Fig. 2

From: Understanding the cluster randomised crossover design: a graphical illustration of the components of variation and a sample size tutorial

Fig. 2

Variation in true mean length(s) of stay (LOS) between intensive care units (ICUs) and between periods within ICUs. Low variation in the true mean LOS between ICUs is shown in the left column (a, c, e, g) and high variation in the right column (b, d, f, h). Low variation in the true mean LOS between periods within ICUs is shown in the top row (a, c, b, d) and high variation in the bottom row (e, g, f, h). a, b, e, f the true mean LOS for each of the 20 hypothetical ICUs are marked by a red circle, with the difference between the true overall mean LOS and the true mean LOS for each ICU indicated by a dashed red horizontal line. The two true cluster-period mean LOS for each ICU are marked with a green circle to the left and right of the true ICU mean LOS. The difference between the true ICU mean LOS and the true cluster-period mean LOS is indicated by a green horizontal line. The black vertical line indicates true overall mean LOS. c, d, g, h the red vertical line indicates the true ICU mean LOS and the green vertical line indicates the true cluster-period mean LOS for each period in each of two ICUs. For (a) WPC = 0.02, BPC = 0.01; for (b) WPC = 0.06, BPC = 0.05; for (e) WPC = 0.06, BPC = 0.01; for (f) WPC = 0.10, BPC = 0.05. ICU 1 is shown with solid lines and ICU 2 is shown in dashed lines in (h). The yellow (blue) curve indicates a normal distribution of patient LOS within each cluster-period where the cluster was allocated to intervention S (T). For (d) the distribution of patient LOS in each of the four cluster-periods are labelled A to D. WPC: within-cluster within-period correlation (ρ); BPC: within-cluster between-period correlation (η)

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