‘Strata’ is a characteristic of stratified sampling method in which the entire population is divided into several homogeneous groups and a simple random sampling technique is applied on each of these groups. Such homogeneous groups are called as Strata.
The population of each and every stratum is identical and significantly differs in its characteristics from those of others. A stratum enables selection of no. of samples in accordance with the proportion of sample population in the entire population. In this way, strata reduce the chances of under or over representation of any particular type of population in sampling and provide proportionate representation to all.
Also, each stratum in itself can be treated as an independent population and can help statisticians to further study the individual stratum in detail.
e.g.) Suppose we want to determine the average income of population of a town through a sampling technique. While taking samples, the entire population is divided into three groups based on the income level – Low, Intermediate and High. Each of these three groups is called as Stratum. The sampling then selects number of people from each of these strata depending upon its proportion in the entire population.