Methodologic issues in follow-up studies of cancer incidence among occupational groups in the United States.

Purpose: Incidence studies of occupational factors and cancer in the United States are problematic because the use of population-based registries to identify cases requires development of historical data on subjects' residences and often severely restricts the time period of follow up. This article describes procedures for addressing these challenges.
Methods: We used data from studies of cancer incidence and mortality among microelectronics industry employees to assess various methods for developing residential histories and the relative informativeness of the two studies.
Results: We developed residential histories for 98% of 99,229 mortality study subjects. Analyses making alternative assumptions about residential histories yielded standardized incidence ratios varying by at most 6%. Use of postemployment residential histories increased person-years by up to 62% and increased the observed number of cancers by up to 28%. The proportion of mortality study person-years included in the cancer incidence study ranged from 40% to 77% among work activity subcohorts. The number of observed cancer cases in the incidence study was 60% higher than the number of observed cancer deaths in the mortality study.
Conclusions: Assumptions about residential history had little impact on validity. Use of information sources with national coverage to develop residential histories increased the incidence study's precision. Despite geographic and temporal restrictions, incidence studies provide more data than mortality studies on cancers with good survival. However, the potential for selection bias in incidence studies may vary considerably among subcohorts, indicating the need for cautious interpretation of such research.