Statistical methods
Dose information from 1951 to 1988 was used in calculating excess relative risk for this cohort. The data were grouped into categories, defined in terms of the following covariates: age (5-year intervals; table 2), calendar year (5-year intervals), occupational group (table 3), time since first exposure (5-year intervals), and cumulative whole body dose (table 1). Person-years at risk and cases were allocated on the basis of these categories and were computed according to the same methods as in the mortality study (3).