What are some of the criteria involved in selecting a representative sample ?

Before selecting a representative sample from a given population the following two main principles should be kept in mind :-

(i) The principles is that all the relevant sub group in the population should be recognised and representations in the sample. Most large populations are not homogenous - They belong to distinct sub-categories. This is called stratfication.

Example : When considering the population of India, we must take account of the fact that this population is divided into rural and urban sectors which are ver different from each other. When considering the rural population of any one state, we have to allow for the fact that this population lives in villages of different sizes. In the some way, the population of a single village may be stratified by class, caste, gender, age religion or other criterion.

(ii) The second Principle of sample selection is that the actual unit–i.e. person or village or household - should be based purely on chance. This is referred to as randomisation, which itself depends on the concept of probability. You may have come across the idea of probability inmathematics course. Probability refers to the chance (or the odds) of an event happening.

Example : When we toss a coin, it can fall with the ‘head’ side up or the ‘tail’ side up. With normal coins, the chance - or probability - of heads or tails appearing is exactly the same, that is 50 percent each. Which of the two events actually happens when you toss the coin – i.e. whether it comes up heads or tails - depends purely on chance and withing else. Even like this are called random events.