Try this for yourself. Suppose you want to estimate the number of field mice in a small field. One day you capture 500 mice, mark all of them and release them. The next week you capture 200 mice of which 40 were marked by you yesterday. What is your estimate of the population size?

The approximate size of the field mice population is 2500 individuals.

Explanation:

There are different methods to study population density. There are direct methods and indirect methods. Among these last ones, we might find the "Mark-Recapture technique".

The Mark-Recapture technique assumes that the population is closed,  during the sampling season there is no mortality nor natality, marks in the individual last the whole sampling season, and these do not affect the marked individual or their behavior. These marked individuals are randomly distributed in the population, and all the individuals have the same probability of being sampled.

The method consists of capturing a sample of individuals belonging to the population under study, in this case, the field mice population. After capturing the individuals, the researcher marks them and release them again. The third step is to sample again, capture a new amount of individuals, and count how many of them are marked. These marked individuals belong to the first sample. We can calculate the population size by using the Lincoln Index or any other index.

The Lincolns Index states that to obtain the population size, the research must follow the following procedure:

1) Take an initial sample with n1 number of individuals

2) Mark all the sampled individuals and release them

3) Take a second sample with n2 number of individuals

4) Count the number of marked individuals m

To estimate the total number of individuals in this population, N, the following equation must be considered:

N = (n1 x n2)/m

In the exposed example, this is:

•n1= 500 individuals in the first sample

•n2= 200 individuals in the second sample

•m= 40 individuals in the second sample that were previously marked  

             as they were part of the first sample    

N = (n1 x n2)/m = (500 x 200) / 40 = 2500 individuals

The approximate size of the field mice population is 2500 individuals.  

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