Valery P C, Williams G, Mcwhirter W, Sleigh A, Scott D, Bain C
Queensland Institute of Medical Research, Queensland, Australia.
Ann Epidemiol. 2000 Nov;10(8):504-8. doi: 10.1016/s1047-2797(00)00070-3.
We report our telephone-based system for selecting community control series appropriate for a complete Australia-wide series of Ewing's sarcoma cases.
We used electronic directory random sampling to select age-matched controls. The sampling has all listed telephone numbers on an up-dated CD-Rom.
95% of 2245 telephone numbers selected were successfully contacted. The mean number of attempts needed was 1.94, 58% answering at the first attempt. On average, we needed 4.5 contacts per control selected. Calls were more likely to be successful (reach a respondent) when made in the evening (except Saturdays). The overall response rate among contacted telephone numbers was 92.8%. Participation rates among female and male respondents were practically the same. The exclusion of unlisted numbers (13.5% of connected households) and unconnected households (3.7%) led to potential selection bias. However, restricting the case series to listed cases only, plus having external information on the direction of potential bias allow meaningful interpretation of our data.
Sampling from an electronic directory is convenient, economical and simple, and gives a very good yield of eligible subjects compared to other methods.
