geographic practice patterns, and the decedent's health history and earlier access to medical care.

Cause of death as documented on a death certificate also varies over time: diagnoses come in and out of favor, and medical technology or knowledge may revise diagnostic criteria and categories. Over the course of the 50 years of mortality follow-up for this study, for example, terminology has been revised regarding lymphosarcoma and various dementias.

The basic comparisons in this study are based on all-cause mortality, which is generally a more accurate measure than cause-specific mortality. As for the specific cause of most interest in this study—leukemia—the reliability of that coding is excellent.

In summary, we found that our mortality ascertainment was very complete and well-balanced between participants and controls. If we were to take NDI and HCFA results as reliable indicators of missing deaths, the impact on our crude mortality would be either to increase participant mortality about 1.2 percent (using HCFA data) or to decrease it by 0.4 percent (using NDI data) relative to controls. Finally, the recoding of mortality causes suggests that any error induced from coding will be very small in comparison to other possible sources of error. We believe these mortality data may be used with confidence.

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