Quota sampling is a method for selecting survey participants that is a non-probabilistic version of
stratified sampling.
Process
In quota sampling, a population is first segmented into
mutually exclusive
In logic and probability theory, two events (or propositions) are mutually exclusive or disjoint if they cannot both occur at the same time. A clear example is the set of outcomes of a single coin toss, which can result in either heads or tails ...
sub-groups, just as in
stratified sampling. Then judgment is used to select the subjects or units from each segment based on a specified proportion. For example, an interviewer may be told to sample 200 females and 300 males between the age of 45 and 60. This means that individuals can put a demand on who they want to sample (targeting).
This second step makes the technique non-probability sampling. In quota sampling, there is non-
random sample
In statistics, quality assurance, and survey methodology, sampling is the selection of a subset (a statistical sample) of individuals from within a statistical population to estimate characteristics of the whole population. Statisticians atte ...
selection and this can be
unreliable. For example, interviewers might be tempted to interview those people in the street who look most helpful, or may choose to use
accidental sampling
Convenience sampling (also known as grab sampling, accidental sampling, or opportunity sampling) is a type of non-probability sampling that involves the sample being drawn from that part of the population that is close to hand. This type of sampl ...
to question those closest to them, to save time. The problem is these samples may be
biased in a way that is difficult to quantify or adjust for. For example, if interviewers decide to question the first person they see, they may oversample tall respondents (who are more easily visible from a distance), which could lead to an
overestimate of average income. This non-random element is a source of uncertainty about the nature of the actual sample.
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Uses
Quota sampling is useful when time is limited, a sampling frame In statistics, a sampling frame is the source material or device from which a sample is drawn. It is a list of all those within a population who can be sampled, and may include individuals, households or institutions.
Importance of the sampling fra ...
is not available, the research budget is very tight or detailed accuracy is not important. Subsets are chosen and then either convenience or judgment sampling is used to choose people from each subset. The researcher decides how many of each category are selected.
Connection to stratified sampling
Quota sampling is the non-probability version of stratified sampling. In stratified sampling, subsets of the population are created so that each subset has a common characteristic, such as gender. Random sampling chooses a number of subjects from each subset with, unlike a quota sample, each potential subject having a known probability of being selected.[ ]
See also
* Coefficient of variation
* Standard deviation
References
*Dodge, Y. (2003) ''The Oxford Dictionary of Statistical Terms'', OUP.
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Sampling techniques
Quotas