Guidelines on Selecting Survey Sample
Sampling is unnecessary if the population to be surveyed is small. However, for larger populations it is costly to perform a full census, and so an unbiased sample of the population should be used.
Size of sample
Generally it is accepted that a sample of 1,500 is the upper limit of any sample size. Beyond this the cost outweighs the benefits.
The rule of thumb in sampling is having a sample size that allows analysis of the important sub-groups of the population. For example, if you are interested in young people's views on drugs you would have a certain sample size, but if you also want to know how different subsets of these young people think, such as those of school age and older, then the sample size would need to be bigger to acommodate both sub-groups. Many analysts for national surveys assume around 100 cases in each sub-group of a sample.
Sample sizes and non-responses
It is important to adjust the number sampled to allow for the expected non-response rate. For example, if the target response rate for a survey is 100 responses, and the estimated non-response rate is 20% then you would send out 125 questionnaire's to get 100 back. The response rate depends upon the level of interest in the subject and incentives used to encourage people to respond.
Types of sampling
There are two main types of sampling:
- randon or unbiased
- purposive
A random or unbiased sample is one in which every member of the wider population has an equal chance of being included in the sample, and so the sample reflects the characteristics of the wider population. You would use this type of sample to understand how the wider customer base is behaving or thinking.
One way of creating a sample is to select a name or address on a systematic basis, so if there are 400 households in an area and the desired sample size is 70 households, then every 6th household on the list would be selected to create the list of sample addresses.
A purposive sample is one that targets a part of the population with specific characteristics of interest, for example, a survey of young adult males between the ages of 18 and 24, who are also unemployed.
The Research and Information team have prepared a guidance note to help you select the relevant sample size when carrying out survey work. See documents section below.
Publications & documents
Contact information
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