Non probability sampling is the sampling procedure in which samples are selected based on the subjective judgment of the researcher, rather than random selection. This is used when the representativeness of the population is not the prime issue. Convenience or judgments of the investigators play an important role in selecting the samples. In general, there are four types of non probability sampling called convenience sampling, judgment sampling, quota sampling and snowball sampling.
The samples are drawn at the convenience of the investigator. The investigator pick up cases which are easily available units keeping the objectives in mind for the study.
· Useful for pilot study.
· Use the results that are easily available.
· Processes of picking people in the most convenient and faster way to immediately get their reactions to a certain hot and controversial topic.
· Minimum time needed and minimum cost incurs.
· High risk of selection bias.
· May provide misleading information.
· Not representative sample. Errors occur in the form of the members of the population who are infrequent or non users of that location and who are not related with the study.
In this type, initial group of respondents are selected. Those respondents are requested to provide the names of additional respondents who belong to the target population of interest. It is a sampling method that involves the assistance of study subjects to identify other potential subjects in studies where subjects are hard to locate such as sex workers, drug abusers, etc. This type of sampling technique works like a chain referral. Therefore it is also called chain referral sampling.
· Appropriate for small specialized population.
· Useful in studies involving respondents rare to find.
· It takes more time
· Most likely not representative
· Members of the population, who are little known, disliked or whose opinions conflict with the respondents, have low probability of being included.
The investigator believes that in his opinion, some objects are the best representative of the population than others. It involves “hand picking” of sampling units. That is the interviewer uses his judgment in the selection of the sample that who should represent the population to serve the investigator’s purpose. It is usually used when a limited number of individuals possess the trait of interest. This type of sampling is also known as purposive sampling. This is useful when selecting specific people, specific events, specific prices of data, etc.
Selecting members for a competition like quiz, oratorical contest to represent a school.
· Low expense.
· Minimum time needed.
· Highly subjective.
· Generalization is not appropriate.
· Certain members of the population will have a smaller chance or no chance of selection compared to others.
· This method does not give representative part of the population, since favoritism is involved.
This is another non-probability sampling method. In this method, the population is divided into different groups and the interviewer assign quotas to each group. The selection of individuals from each group is based on the judgment of the interviewer. This type of sampling is called quota sampling. Specified sizes of number of certain types of peoples are included in the sample.
· The selection of the sample in this method is quick, easy and cheaper.
· May control sample characteristics.
· More chance of representative.
· Selection bias.
· The sample is not a true representative and statistical properties cannot be applied.
A selection committee wants to compose a cricket team (11 players) for a test match.
In the composition of a cricket team, the selection committee forms groups compartmentalize as pace bowlers, spinners, all-rounders, batsmen and wicket keepers. The committee fixed quota for each group based on the pitch and the opponent teams’ strength. Then, from each group they select the required number of players using judgement. Look the table.