Define and explain the major types of non-probability sampling.
What will be an ideal response?
Answer:
Nonprobability sampling methods are those in which elements within the population do not have a known probability of being selected into the sample. Since the chance of any element being selected for the sample is unknown, there is no certainty that the selected sample represents the population accurately. Nonprobability sampling methods are commonly used when there is no population list, the study is qualitative in nature, or the population is relatively small. There are four types of nonprobability sampling methods: availability sampling, quota sampling, purposive sampling, and snowball sampling.
Availability sampling, also known as convenience sampling, selects elements based on their convenience. Examples of this are standing on a street corner and talking to those who walk by, asking questions to employees to come pick up their check as a personnel office, or distributing questionnaires to an available audience. Essentially, the given sample is selected for no other reason that such elements are easily accessible tothe researcher.
Quota sampling selects elements to ensure that the sample represents certain characteristics in proportion totheir prevalence in the population. Quotas are set to ensure that the sample represents certain characteristics in proportion to their prevalence in the population. Similar to stratified proportional sampling, quota sampling methods differ in that the selection process for elements is less rigorous and precise.
Purposive sampling selects elements for a purpose, usually because of their unique position. Often called judgement sampling, researchers use their own judgement about whom to select into the sample rather than drawing sample elements randomly. An example of a purposive sample is studying a subset of the parolee population, such as juvenile parolees at a specific facility.
Snowball sampling occurs when sample elements are selected as they are identified and recruited by successive elements, informants, or interviewees. In essence, one element identifies another, who may identify two other elements, who may identify n number of additional elements in a cycle until the desired sample size isachieved. An example of snowball sampling is illicit drug users, where one is willing to identify other drug users he/she knows to be a part of the study who in turn identifies other drug users that are willing.
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