A systematic error in tests of ideal free theory

Classical ideal free theory predicts that the distribution of consumers within a patchy environment should correspond to the distribution of resources. Tests of this prediction have inappropriately compared ratios of mean resource levels and mean consumer densities, rather than means of ratios. We show that this error, which has propagated through hundreds of studies, leads to a systematic bias: the theory will appear to underestimate the number of consumers occupying poor patches. We explain the correct way to test ideal free theory and apply it to published data; the classical model is then seen to yield far more accurate predictions than previously thought.