Biological systems: recognition of covariance.
'Hold all variables constant except one', is not a feasible approach to the study of the many biological systems in which variables are not independent, but covariant. Covariance means that the efficacy of a variable depends on the values of other variables. The presence of covariance in biological systems must be recognized in the design of experiments and the analysis of results.
D000704:Analysis of Variance; D000818:Animals; D001695:Biology / Q000379:methods*; D006801:Humans; D008099:Liver / Q000196:embryology / Q000201:enzymology