3.1.8 Non-probability sampling Methods
In this method of sampling the selectuon of sample items depends exclusively on the judgement of the investigator. In other words, the investigator exercises his judgement in the selection and includes those items in the sample which he thinks are most typical of the universe with regard to the characteristics under investigation. For example, if sample of ten students is to be selected from a class of sixty for analyzing the spending habits of students, the investigator would select 10 students who, in his opinion, are representative of the class.
Merits: Though the principles of sampling theory are not applicable to judgment sampling, the method is sometimes used in solving many types of economic and business problems. The use of judgment sampling is justified under a variety of circumstances.
i. when the number of units in the population, simple random selection may miss to include the more important elements, whereas judgment sampling would certainly include them in the sample.
ii. When some unknown traits of a population are to be studied and some of whose characteristics are known, we may then stratify the population according to these known properties and select sampling units from each stratum on the basis of judgement. This method is used to obtain a more representative sample.
iii. In solving everyday business problems and making public policy decisions, executives and public officials are often pressed for time and cannot wait for probability sample designs. Judgment sampling is then the only practical method to arrive at solutions to their urgent problems.
Limitations. i. This method is not scientific because the population units to be sampled may be affected by the personal prejudice or bias of the investigator. Thus, judgment sampling involves the risk that the investigator may establish foregone conclusions by including those items in the sample which conform to his preconceived notions. For example, if an investigator holds the view that the wages of workers in a certain establishment are very low, and if he adopts the judgment sampling method, he may include only those workers in the sample whose wages are low and thereby establish his point of view which may be far from the truth. Since an element of subjectiveness is possible, this method cannot be recommended for general use.
ii. There is no objective way of evaluating the reliability of sample results
The success of this method depends upon the excellence in judgment. If the individual making decisions is knowledgeable about the population and has good judgment, then the resulting sample may be representative, otherwise the inferences based on the sample may be erroneous. It may be noted that even if a judgment sample is reasonably representative, there is no objective method for determining the size or likelihood of sampling error. This is a big disadvantage of this method.