Non-Probability Sampling Methods
- Purposive Method of Sampling: This method of sampling is also otherwise known as subjective, or ‘judgment sampling’ method. Under this method of sampling the investigator himself purposively chooses certain items which according to his judgment are the best representatives of the universe. Here the selection is deliberate, and is based on own idea of the investigator about the sample units. As such under this method, the chance of inclusion of some items in the sample is very high, while that of the others is very low. However, for better selection of the items under this method certain criteria of selection are first laid down and then the investigator is allowed to make the selection of the items of his own accord within the orbit of those criteria.
- Quota Sampling: It is a type of purposive sampling in which the whole universe is divided first into certain parts, and the total sample is allocated among these parts. Each part of the population is assigned to an investigator for whom the quota of the units to be examined by him is fixed in advance according to certain specified characteristics such as sex, age, occupation, income group, political, or religious affiliation. The investigator is asked to select the required number of units of the sample of his own accord, and examine them to get the desired information as quickly as possible. He is also authorized to substitute the new units in the quota, if he finds that any unit of the sample so selected is not responding up to the mark. This method is very often used in opinion poll surveys, market surveys and political surveys etc.
- Convenience Sampling: It is a type of purposive sampling in which the sample units are selected purposively by the investigator to suit his convenicence in the matter of location, and contact with the units. This method of selecting the sample is also called ‘Chunk’ since the samples under this method are selected neither on the investigator but on the basis of convenience on the part of the investigator. For examples, a sample obtained for ma list of students in a college for enquiring into their educational problems is a matter of convenience sampling as in such a case, it for obtaining the desired information from them.
