Re: Mortality among workers in the geothermal power plants at Larderello, Italy. Am. J. Ind. Med. 35:536–539, 2000*

Abstract In a cohort mortality study, a few basic rules must be respected for risk estimation including: (1) the definition of the criteria of entry in the cohort; (2) the time period of observation; (3) the outcome, which must be the same for observed and expected events [Breslow and Day, 1987; MacMahon and Trichopoulos, 1996]. In their letter to the Editor referring to our article "Mortality Among Workers in the Geothermal Power Plants at Larderello, Italy" [Pira et al., 1999], Merler et al. appear to ignore these basic principles, since they extend the observation period from 1990 to 1998 for observed but not for expected events and do not distinguish between incidence and mortality. Consequently, their relative risk estimates are simply not interpretable. Anyway, an update of the follow‐up to the end of 2000 has already been planned.