Captures recapture sampling: Another approach to a census for estimating the size of population, which operates by sampling the population number of times, identifying the individuals which appear more than once. It was first used by Laplace to estimate the population of France; this approach received its main impetus in to context of estimating the approximate size of the wildlife populations. An initial sample is attained and the individuals in that sample are marked or otherwise identi?ed. A next sample is, subsequently, independently attained, and it is noted how many individuals in that particular sample are marked. If the next sample is representative of the population as a whole, then the sample proportion of the marked individuals should be about the same as that of the corresponding population proportion. From this relationship total number of individuals in the population can be estimated. Particularly if X individuals are 'captured', marked, released and y individuals then the independently captured of which x are marked, then the estimator of the population size (often called as the Petersen estimator)is given as follows

 
with variance given by
 

The estimator does not have the ?nite expectation since x can take zero value. A modi?ed version, Chapman's estimator, adds one to the frequency of the animals caught in both the samples (x)with the follow-on population size estimator