looks only for large creatures and the other only for small), or in other circumstances. Limitations The Lincoln Index is merely an estimate. For example, the species in a given area could tend to be either very common or very rare, or tend to be either very hard or very easy to see. Then it would be likely that both observers would find a large share of the common species, and that both observers would miss a large share of the rare ones. Such distributions would throw off the consequent estimate. However, 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. See also Sampling Theory Drake equation German tank problem Inter-rater reliability Further reading Petersen, C. G. J. (1896). "The Yearly Immigration of Young Plaice Into the Limfjord From the German Sea", Report of the Danish Biological Station (1895), 6, 5–84. Notes Ecological techniques Estimation methods Environmental statistics