The COCON (COhort CONtraception) survey on contraception, unplanned pregnancy and induced abortion in France, was set up to interview by telephone a representative sample of women aged 18-44, and to follow them up annually over five years. Between 2000 and 2002, the survey sample fell in size by one third, from 2,863 to 1,912 women. This article describes this attrition process and evaluates the biases that it causes in the analyses. The attrition is due, in roughly equal proportions, to two factors: first, an inability to re-contact the women; second, refusals to be re-interviewed. It is highly selective, concerning primarily foreign women, of low educational level, young and living alone, and was responsible for modifying the structure of the sample followed up and the means for several survey variables of interest. However, the attrition has little impact on the multivariate analyses. Finally, the implications of attrition in COCON are limited as regards the biases caused, and are seen mainly as the loss of precision in the estimates due to the reduction in sample size.
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