For more than a century ecologists have sought to explain the spatial heterogeneity of plants and animals1, but progress has been hampered by measurement bias2. A measure thought to be an unbiased index of spatial heterogeneity3 is b, the fitted exponent in the empirical relationship s2 = aMb, where s2 is the variance and M the average of randomly placed replicate population estimates 4. This index is widely accepted because of the impressive correlation between s2 and M and because it requires no inter-organism distance measures. Theoretical models, based on migratory behaviour5 or demographic factors6, have been proposed to account for the relationship between s2 and M. These models disagree regarding the effect of environment on b and the divergence of b values shown by different species. Here I report data showing that species-specific b varies among environments and that different species often show similar b values, favouring the demographic model. Analysis of these data shows that different levels of replication and sampling coverage lead to biased b values, casting doubt on the use of b for the deduction of cause of spatial variance relationships or the comparative measurement of spatial heterogeneity.