There are various different ways to collect information from human populations. Each method has its advantages and disadvantages.

Telephone

Mailed questionnaire

Interviewer

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 to match the corresponding proportion in the target population.

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.