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Random Sampling

The term random doses not mean haphazard, careless, or unplanned, but it means that selection of items must be controlled in such a manner that every items has an equal chance of being included in the sample. The units are not chosen on accidental basis. It is carefully planned. It gives each unit in the population a calculable probability of being selected. Therefore random sampling is also known as probability sampling. The selection is free from the bias and prejudice of the investigator because he does not exercise his discretion and preference in the selection of the items. Random sample is a representative sample. It properly selected it would represent all the groups in the Universe. The sampling theory rests on the principle of randomness in the selection procedure. Random samples can be obtained by the following method.

(a)    Lottery Sampling: In this procedure all the items of the universe are numbered or named on separate slips of paper. These slips are folded and placed in a suitable container and mixed up properly. After determination of the sample size, the required number, if our of 500,100 items have to be drawn, the slips will be properly mixed and the first 100 slips drawn out of the container will constitute the sample. The selection of items depends on change. It ensures equi-probability of selection.

In this procedure, the tossing, mixing and drawing is important. The slips should be thoroughly shuffled after each selection of item to ensure randomness. This mixing is repeated till the entire sample is obtained.

A few precautions must be taken while listing and numbering the elements. The slips should be of equal size, shape and colour. If the slips are not of equal size, the bigger slips will tend to remain at the top, with the result that all the items will not have an equal opportunity of being included in the sample. Secondly of being included in the sample. Secondly, all the items of the universe should item. The list of universe can be obtained form many sources, depending on the object of the study such as telephone directories, voting registrations, automobile owners list, distribution of welfare services, juvenile offences etc.

Each procedure has its own special qualifications conditions and limitations. It the universe is large, the listing and numbering of the population becomes laborious. Moreover if it is not possible to list all the units and if the list is incomplete and inaccurate, it is not possibility of selection. Further more it is difficult to construct perfect coins, perfect balls or prefect slips, or perfect cards and to bring about complete mixing.

(b)Table of random numbers: A superior method is the use of random table numbers. Many types of random numbers are available.
(i)    Tippett table of random numbers.
(ii)    Fisher & Yates numbers.
(iii)    Kendall & Babington Smith numbers.

The most commonly used numbers are Tippett’s table of random numbers. Tippett had taken these numbers from census reports. These numbers have been used in many investigations and have found to be reliable.

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