What is a non-probability sampling?
Non-probability sampling derives its control from the judgement of the investigator. In non-probability sampling, the cases are selected on bases of availability and interviewer judgement. Non-probability sampling has its strength in the area of convenience.
Methods of non-probability sampling
The important non-probability sampling methods include
- Convenience sampling
- Quota control sampling; and
- Judgment sampling.
Convenience sampling is generally known as careless, unsystematic, accidental or opportunistic sampling. The sample is selected according to the convenience of the sample. The researcher selects certain units convenient to him. It requires no pre-planning for the selection of items. Convenience sampling ensures convenience in respect of availability of source list and accessibility of the units. Despite being unscientific, a large number of samples are convenient sampling.
A convenience sampling is used in the following situations:
- when the universe is not clearly defined
- where sampling unit is not clear and
- when a complete source list is not available.
Quota sampling combines the features of purposive sampling and stratified sampling. Under quota sampling, the field workers include only those units which conform to certain specified parameters in the sample. Each field worker is assigned quotas of number of units to include according to one or more characteristics. In order to increase the representativeness of a quota sample, the field worker may be instructed to contact every fourth house and interview one person till the quota is fulfilled.
Advantages of Quota sampling
1. Quota sampling ensures convenience in executing sampling study.
2. When the respondent refuses to cooperate, he may be replaced by another person who is ready to furnish information.
3. Quota sampling is less expensive and speedy
4. When the population has no suitable frame, quota sampling is the only practical method.
5. Collection of data through Quota sampling method is not a time consuming one.
Disadvantages of Quota sampling
1. The interviewer interviews people who are easily available and accessible. So, the possibility of collecting valuable data is affected in Quota sampling.
2. Bias arises in the matter of selection of sample units.
3. The work of the interviewer cannot be supervised properly. So, there is no certainty of correctness of data.
4. Quota sampling method requires several investigators. Each one cannot be equally competent. So, the results derived from the study may not be uniform.
Judgement sampling is one of the non-probability methods of sampling. Judgement sampling involves the selection of a group from the population on the basis of available information. It is the selection of the group by intuition on the basis of criteria deemed to be self evident. Under this method, units are included in the sample on the basis of the judgement that the units possess the required characteristics to qualify as representatives of the population.
Advantages of Judgement sampling
The chief advantages of the judgement sampling are as follows:
1. Judgement sampling eliminates 5e cost and time in preparing the sample
2. Judgement sampling method enables the researcher to include the positive aspects of stratification in the sample.
Disadvantages of judgement sampling
1. There is uncontrolled variability and bias in the estimates in Judgement sampling.
2. The success of Judgement sampling method is solely dependent on a thorough knowledge of the population and elimination of the use of inferential parametric statistical tools for the purpose of generalization.
3. Complete reliance on intuition and hunch is risky in Judgement sampling.