Non-Probability Sampling
For quantitative surveys, probability sampling
should be our preferred approach where possible.
It allows randomness to drive the selection and
allows estimates of the accuracy of survey findings
to be obtained. The most likely situation for non-probability
sampling to be needed is when there is either no
sampling frame or the population is so widely
dispersed that cluster sampling would be too
inefficient. Non-probability techniques are cheaper
than probability sampling, and are often used in
exploratory studies e.g. for hypothesis generation.
There are five main non-probability sampling
techniques;
- Purposive Sampling
- Quota Sampling
- Convenience Sampling
- Snowball Sampling
- Self-Selection
1) Purposive Sampling
Purposive sampling is a method where the
participants are selected by the researcher
subjectively. The researcher will pick a sample
that he/she believes is representative to the
population of interest. Respondents are
not selected randomly but by using the judgement of
the interviewers.
2) Quota Sampling
Quota Sampling is perhaps most commonly used in
face-to-face interviewing. Interviewers on the
street are usually looking for a specific type of
respondent – age, gender are the most frequently
used 'quota controls'. Quotas are given to interviewers and are
organised so that the final sample is representative
of the population. It is impossible to estimate the
accuracy of the sample because it is not random.
3) Convenience Sampling
Similar to quota sampling, convenience sampling is a
technique often used in face-to-face interviewing.
A convenience sample is when the interviewer simply stops anyone
in the street or knock on doors asking anyone to
participate and interviewing anyone willing to
help. It is hard to draw any meaningful conclusions
from the results obtained due to the lack of
randomness, meaning the likelihood of bias is high.
4) Snowball Sampling
This approach is often used when trying to interview
hard to reach groups such as unemployed people or
Black or Minority Ethnic residents.. You initially contact a
few potential respondents, interview them and then
ask if they know of anybody else with the same
characteristics you are looking for.
5) Self-Selection
This technique is self-explanatory – respondents
themselves decide whether to take part in the survey
or not.
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