The Lincoln Index provides a way to measure population sizes of individual animal species. It is based on a capture /mark / recapture method.
In theory, mark / recapture techniques involves sampling a population of animals and then marking all of the individuals captured in a recognizable way. The marked animals are then released back into the population and left to mingle for a suitable period of time. Once they have become mixed into the population again, the population is resampled.
The formula for expressing this is the following:
n1xn2/n3

n1
number of animals first marked and released.umber of animals first marked and released.
n2
number of animals captured in the second sample
n3
number of marked animals in the second sample