Assessing effects of occupational exposure on fertility with indirect standardization

The use of indirect standardization in the assessment of the fertility of occupationally exposed workers is briefly reviewed and critiqued. The calculation of expected births in the method of Levine et al. (J Occup Med 1980;22:781-91) is modified to eliminate negative bias. An example is given using data from a 1977 survey of 60 male workers at a chemical manufacturing plant in Denver, Colorado, who were exposed to dibromochloropropane. The example illustrates how in-plant nonexposed reproductive experience provides a valuable supplement to US fertility tables which are specific only to race, birth cohort, age, and parity. It is also shown, however, that explicit control for potential confounding factors not included in the tables, such as marital status and surgical sterilization, can actually create rather than alleviate confounding error. This occurs when the additional factors co-vary in the reference population with the factors already included in the tables. For martial status, the control-induced error was readily minimized by restricting analysis to married experience at parity one or greater. For surgical sterilization, the corresponding error could not be reduced without severely compromising sample size, and hence control of this potential confounder in similar circumstances is not recommended.