Methods of obtaining sample information

Whatever sampling scheme is used, information must be obtained from each individual selected in the sample. When sampling items produced by a factory or trees in a forest, the process of obtaining measurements from each item is usually fairly straightforward.

However there are various options for collecting information from human populations. Each method has its advantages and disadvantages.

Telephone

Telephone surveys are relatively cheap to conduct and, therefore, sample sizes can be greater.

Telephone numbers may be selected at random from a telephone book but this misses unlisted numbers, so random dialling is often used.

Mailed questionnaire

In a postal survey, individuals would be randomly selected from electoral rolls or other population lists. Alternatively, questionnaires might be delivered by hand to the mailboxes of a sample of houses.

Interviewer

Interviewers who approach respondents at home are most likely to get responses for long questionnaires.

Houses are rarely selected at random. Often streets are randomly selected and every 5th or 10th house in the street is approached. This is called a systematic sample.

Street corner

Some surveys are conducted by approaching people in busy shopping centres or similar public places.

To reduce coverage errors, a quota sample is often used. Each interviewer is told to interview fixed numbers of old, young, male, female, etc. respondents. The proportion with each characteristic is chosen to reflect the corresponding proportion in the target population. It is much harder to assess the accuracy of quota samples than the sampling schemes that were described at the start of this section.

Self-selected

Phone-in or mail-in surveys are often conducted by radio stations and magazines. The respondents are usually so unrepresentative that the results are meaningless. These types of survey should be avoided.