Who Makes Sociology's Data? Problems Posed by Secondary Analysis of National Sport Club Membership Files: The secondary analysis of national files of sport club membership is a possible research technique in investigating careers in sports. But before engaging in such work, one must know if the stored data can be used for scientific research. In this framework, we examined the yearly membership files of the Fédération Française de Voile. This critical analysis had two aspects. First, the information on members (wind-surfing or not, competitions or not) was compared with that provided by another high-quality source. This comparison reveals major differences. To interpret the meaning of these differences, we reconstructed the means by which the data was produced for the national files. This work permitted us to evaluate the pertinence of the two recorded variables for eventual longitudinal studies. The result is that the data could be used in longitudinal studies with only slight confidence. The data tends to describe the margins of interpretation left to local authorities by the federal guidelines, and not the real activity of members.