It isn't always possible to undertake a probability method of sampling, such as in random sampling. For example, there is not a complete sampling frame available for certain groups of the population e.g. the elderly; people who are attending a football match; people who shop in a particular part of town. Another factor to bear in mind is that many of the probability sampling methods described above may mean that researchers would have to undertake a postal or telephone survey delivery or might be expected to go from house to house. We will discuss some of the problems of low response rate later on in this workbook, but you might find that a probability sample with a poor response rate doesn't in the end give you a particularly good representation of the population being examined.
Advantages of non-probability methods:
A purposive sample is one which is selected by the researcher subjectively. The researcher attempts to obtain sample that appears to him/her to be representative of the population and will usually try to ensure that a range from one extreme to the other is included.
Often used in political polling - districts chosen because their pattern has in the past provided good idea of outcomes for whole electorate.
Have you ever been ambling along your local High Street, noticed a Market Researcher with a clipboard and thought "I don't mind being asked some questions - it might be interesting", only to find that the researcher looks straight through you? No? Well, for those people who have had that happen, there is no need to take it personally. It is all due to quota sampling.
Quota sampling is often used in market research. Interviewers are required to find cases with particular characteristics. They are given quota of particular types of people to interview and the quota are organised so that final sample should be representative of population.
Stages:
Disadvantage of quota sampling - Interviewers choose who they like (within above criteria) and may therefore select those who are easiest to interview, so bias can result. Also, impossible to estimate accuracy (because not random sample)
A convenience sample is used when you simply stop anybody in the street who is prepared to stop, or when you wander round a business, a shop, a restaurant, a theatre or whatever, asking people you meet whether they will answer your questions. In other words, the sample comprises subjects who are simply available in a convenient way to the researcher. There is no randomness and the likelihood of bias is high. You can't draw any meaningful conclusions from the results you obtain.
However, this method is often the only feasible one, particularly for students or others with restricted time and resources, and can legitimately be used provided its limitations are clearly understood and stated.
Because it is an extremely haphazard approach, students are often tempted to use the word "random" when describing their sample where they have stopped people in the street, as they see it "at random". You should avoid using the word "random" when describing anything to do with sampling unless you are absolutely certain that you selected respondents from a sampling frame using truly random methods.
With this approach, you initially contact a few potential respondents and then ask them whether they know of anybody with the same characteristics that you are looking for in your research. For example, if you wanted to interview a sample of vegetarians / cyclists / people with a particular disability / people who support a particular political party etc., your initial contacts may well have knowledge (through e.g. support group) of others.
Self-selection is perhaps self-explanatory. Respondents themselves decide that they would like to take part in your survey.
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