The Lincoln index is a statistical measure used in several fields to estimate the population size of an animal species.
A similar example involves estimating the number of typographical errors remaining in a text, from two proofreaders' counts.
This can arise if the observers only find a small percentage of the actual species (perhaps by not looking hard enough or long enough), if the observers are using methods that are not statistically independent (for example if one looks only for large creatures and the other only for small), or in other circumstances.
Bagaimanapun, such distributions are unusual for natural phenomena, as suggested by Zipf's Law.
T. J. Gaskell and B. J. George propose an enhancement of the Lincoln Index that claims to reduce bias.