Is probability sampling always better? A comparison of results from a quota and a probability sample survey.

Two surveys in the same defined population in Sydney's western suburbs in 1986 and 1987 provided the opportunity to compare results obtained from a quota and a probability sample survey. These surveys were designed to provide information for the planning of local health promotion programs. The quota sample survey was conducted in shopping centres and used quota sampling to select 1727 respondents. In the second survey, area probability sampling was used to select 484 respondents. This survey had a response rate of 65 per cent. There were 15 questions common to both surveys; results of only three differed significantly (p less than 0.05) between surveys. None of these differences was important from a public health perspective. The agreement between the results of these two surveys probably reflects the fact that the same selection bias has operated in both. Unless a very high response rate can be achieved, quota sample surveys with age and sex quota controls may be an acceptable alternative to probability sample surveys for gathering local data relevant to the development of health programs.