Palaeozoological data are asymmetrical because they indicate the presence of a species in the area in which its remains are recovered, but the absence of remains is not necessarily evidence for the absence of a species. It is impossible to measure the magnitude of data asymmetry with respect to biogeography, but the magnitude can be estimated. The proportion of sites in an area where a species is known to occur that have not produced remains of that species is an estimate of counter-ubiquity. The proportion of identified faunal remains in an area where a species is known to occur that do not represent that species is another estimate of data asymmetry. Bivariate plots of the number of sites in an area that have produced remains of a taxon against the total number of faunal remains identified in the area indicate that data asymmetry can be estimated as the inverse of sample size. More sites and more identified faunal remains tend to produce more occurrences of a species, so more sites and more identified faunal remains will tend to provide a database that is not only more accurate but also less asymmetrical with respect to the geographical range of a species. Copyright © 2007 John Wiley & Sons, Ltd.