Random sampling has already been discussed. This post deals with non-random sampling which is a sample that is selected in a deliberate manner. Below are a few of the more common forms of non-random sampling
- Convenience sampling is the selection of individuals who are available for the study. Whoever is free and willing is a part of the study.
- Purposive sampling is the inclusion or participants based on a criteria developed by the researcher. For example, a researcher wants to only include middle age male teachers in their study. Individuals who meet this criteria will be asked to be a part of the study.
- Quota sampling is used when a researcher collect data from a certain number of people from several sample units or sub-populations who meet certain criteria. For example, at a university, selecting students from several different departments such as English and Education to be a part of the study. Whoever is from either department may be a part of the study
- Snowball sampling is a technique in which the researcher locates one member of the sample population and collects data from them. The participant then recommends other people the researcher can collect data from. An example, would be a detective interviewing various people about a crime. One witness suggest someone else the detective should talk to. This form of sampling is common in qualitative research.
Sampling is normally influenced by circumstances. Random is often preferred to non-random both the context often dictates that a researcher do the best they can and choose a technique that is appropriate for the situation.