A decision matrix to better identify repeatable physiological variation within individuals

The performance of an individual has remained at the heart of evolutionary biology since the time of Darwin. Physiologists are equally drawn to the implications of individual variation for health and sporting endeavours, and specifically, whether or not a physiological trait is repeatable within an individual. Experimental biologists are especially interested in temporally stable physiological traits that are relevant to an individual's lifetime fitness for natural selection to act upon. Experimental noise, however, confounds the measurement of such repeatability, even though validated protocols exist for measuring many meaningful physiological performance traits. Missing is a decision matrix that helps distinguish individual variation from experimental noise. We propose a Precision-&-Repeatability Assessment Matrix (PRAM) that integrates established assessments of individual variability and repeatability. This matrix places metrics that are more repeatable and precise in the quadrant closest to the origins of Cartesian coordinates; those farthest away are less acceptable in terms of both repeatability and precision. As a case study, PRAM is applied to whole-organism aerobic and non-aerobic metabolic performance metrics from fish that were measured with the same protocols. The analysis illustrates that aerobic metabolic metrics can be more repeatable and precise than non-aerobic ones. Consequently, PRAM helps physiologists to better understand whether the observed variability is owing to non-repeatable metrics or true individual variation. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.