• analysis of variance;
  • ancova;
  • condition factor;
  • regression analysis;
  • statistical analysis


  • 1
    An analysis of variance (anova) or other linear models of the residuals of a simple linear regression is being increasingly used in ecology to compare two or more groups. Such a procedure (hereafter, ‘residual index’) was used in 8% and 2% of the papers published during 1999 in the Journal of Animal Ecology and in Ecology, respectively, and has been recently recommended for studying condition.
  • 2
    Although the residual index is similar to an analysis of covariance (ancova), it is not identical and is incorrect for at least four reasons:
  • (i)
     the regression coefficient used by the residual index differs from the one used in ancova and is not the least-squares estimator of the model.
  • (ii)
    in contrast to the ancova, the error d.f. in the residual index are overestimated because of the estimation of the regression coefficient.
  • (iii)
    the residual index also assumes the homogeneity of regression coefficients (parallelism assumption), which should be tested with a special ancova design.
  • (iv)
    even if the assumptions of the linear model hold for the original variables, they will not hold for the residuals.
  • 3
    More importantly, the residual index is an ad hoc sequential procedure with no statistical justification, unlike the well-known ancova. For these reasons, I suggest that a t-test or an anova of the residuals should never be used in place of an ancova to study condition or any other variable.