The follow-up of Gazel volunteers is based on the fact EDF & GDF agents form a "captive" population, whom systematic follow-up of workers is done by these companies, both during their career and after retirement.

The particular status of EDF & GDF employees makes people who begin to work there stay for their whole career. Since a pension plan is provided by the companies, the administrative link continues even after retirement. Besides, EDF & GDF agents do not leave the company when they stop work for a long illness or infirmity ; there is no withdrawals for health reasons, which avoids a well known source of bias, the healthy worker effect.

Even if leaving EDF & GDF is exceptional, tranfers and moving do occur frequently, but this does not affect the follow-up.

Finally, the only real Gazel withdrawals are people who resign from EDF & GDF or ask to leave the cohort. This happens rarely and the rate of withdrawal will probably stay insignificant.