Question

Explain the stratifi ed random sampling with a suitable example

Answer 1

Stratified Random Sampling

In stratified random sampling, first divide the population into sub-populations, which are called strata.

Then, the samples are selected from each of the strata through random techniques.

The collection of all the samples from all strata gives the stratified random samples.

When the population is heterogeneous or different segments or groups with respect to the variable or characteristic under study, then stratified Random sampling method is studied.

First, the population is divided into homogeneous number of sun-groups of strata before the sample is drawn.

A sample from each stratum at random.

Following steps are involved for selecting random sample in a stratified random sampling method.

(a) The population is divided into different classes so that each stratum will consist of more or less homogeneous elements.

The strata are so designed that they do not overlap each other.

(b) After the population is stratified, a sample of a specified size is drawn at random from each stratum using Lottery Method or table of random number method.

Stratified random sampling is applied in the field of the different legislative areas as strata in election polling, division of districts (strata) in a state, etc…

Example: From the following data, select 68 random samples from the population of the heterogeneous group with a size of 500 through stratified random sampling, considering the following categories as strata.

• Category 1: Lower-income class – 39%
• Category 2: Middle-income class – 38%
• Category 3: Upper-income class – 23% 

Stratum	Homogeneous group	Percentage from population	Number of people in each stratum	Random Samples
Category 1	Lower-income class	39	`39/100 xx 500` = 195	`195 xx 68/500 = 26.5 - 26`
Category 2	Middle-income class	38	`38/100 xx 500` = 190	`190 xx 68/500 = 26.5 - 26` `190 xx 68/500 - 26.5 - 26`
Category 3	Upper-income class	23	`23/100 xx 500` = 115	`115 xx 68/500 = 15.6 - 16`
Total		100	500	68