• accuracy;
  • metabolism;
  • mixed-effect model;
  • multi-response model;
  • personality;
  • physiology;
  • plasticity;
  • random regression;
  • reaction norm;
  • repeatability;
  • statistical power


  1. Growing interest in proximate and ultimate causes and consequences of between- and within-individual variation in labile components of the phenotype – such as behaviour or physiology – characterizes current research in evolutionary ecology.
  2. The study of individual variation requires tools for quantification and decomposition of phenotypic variation into between- and within-individual components. This is essential as variance components differ in their ecological and evolutionary implications.
  3. We provide an overview of how mixed-effect models can be used to partition variation in, and correlations among, phenotypic attributes into between- and within-individual variance components.
  4. Optimal sampling schemes to accurately estimate (with sufficient power) a wide range of repeatabilities and key (co)variance components, such as between- and within-individual correlations, are detailed.
  5. Mixed-effect models enable the usage of unambiguous terminology for patterns of biological variation that currently lack a formal statistical definition (e.g. ‘animal personality’ or ‘behavioural syndromes’), and facilitate cross-fertilisation between disciplines such as behavioural ecology, ecological physiology and quantitative genetics.