If the population is hetrogeneous, then simple random sample cannot be a representative sample of the population. In this situation, by using the stratified random sampling stratified random sample is be selected.

In this method, first of all units of a population are divided into different strata of homogeneous nature. All these strata are different among themselves but are internally homogeneous.

Now, a random sample is taken from each stratum and random samples so obtained from all strata are combined to get a sample which is called a stratified random sample and the method of selecting such a sample is called stratified random sampling method.

Illustration: Suppose out of 600 families in an area of a city, 200 families are having less Income, 300 families are having average Income and 100 families are having more Income. For socio-economic survey of these families sample of 60 families is to be selected. Taking the families of different income groups as strata families are randomly selected from each stratum in the proportion of 200:300:100. In this way, 20 families having less income, 30 families having average income and 10 families having more. Income are selected at random and the sample of 60 families so obtained is known as stratified random sample.

In stratified random sampling :

• The population is divided into different strata so each stratum gets representation. Hence the representative sample can be obtained for all the characteristics of the population.
• The work of selecting samples from different strata can be assigned to separate investigators or enumerators and hence administrative convenience can be improved.
• If proper care is not taken while dividing the population into different strata, then appropriate representative sample of the population cannot be obtained and therefore the reliability of results can be reduced.