In all the methods discussed so far, the result of the sampling process is a list of all those to be interviewed. The interviewers must then contact these sampling units, and this may take a considerable amount of time. It is possible that, in spite of every effort, they may have to record "no contact" on their questionnaire. This may lead to a low response rate and hence the survey result would be biased and a great deal of effort, time and money would have been wasted. To overcome these problems the method of quota sampling has been developed, in which a sampling frame and a list of sampling units is not necessary This is an example of a non- probability sampling method, because it is not possible to determine the probability that any individual member of the population will be included in the sample. The basic difference between this method and those we have already discussed is that the final choice of the sampling units is left to the sampler in person.
The organisers of the survey supply the sampler, usually an interviewer, with the area allocated to him or her and the number and type of sampling units needed. This number, called a quota, is usually broken down by social class, age or sex. The interviewers then take to the street and select the units necessary to make up their quota. This sounds simple but in reality selecting the quota can be difficult, especially when it comes to determining certain characteristics like the social class of the chosen person. It requires experience and well- trained interviewers who can establish a good relationship quickly with those people being interviewed.
... Advantages – the advantages of this method are that it is probably the cheapest way of
collecting data; there is no need for the interviewers to call back on any respondent,
they just replace any respondent with another more convenient to locate; it has been
found to be very successful in skilled hands.
... Disadvantages – the disadvantages are that as the sample is not random, statistically
speaking, it is difficult to assess a degree of confidence in the deductions; there is too
much reliance on the judgement and integrity of the interviewers and too little control
by the organisers.
Judgemental Sampling
Judgemental sampling is a non-probability sampling method in which the researcher uses his or her judgement to choose appropriate members of the population for the sample. Often, the sample members are selected because they are thought to be experts in the field who can provide useful information on the research topic.
Snowball Sampling
Snowball sampling is a non-probability sampling method in which a small sample is first selected (using, for example, either random or judgemental sampling) and then each sample member is asked to pass on a questionnaire to acquaintances. In this way, a much larger sample can be obtained.
Convenience Sampling
Convenience sampling is a non-probability sampling method in which the sample members are selected because of their availability and willingness to participate. For example student researchers, who are collecting primary data for a dissertation, may collect data from their
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