PURPOSE: Estimating the effects of continuous chronic disease risk factors on mortality is an area that generates confusion and controversy. The frequently observed U-shaped or J-shaped relationships between the risk factors and mortality are often in contrast with presumed monotone relationships. Therefore, some investigators suggest that subjects dying during the first k years of follow-up (where k is some positive number less than the total length of follow-up) be excluded from statistical analyses. The rationale for this approach is that subjects dying during the first k years of follow-up are likely to have some pre-existing occult disease that confounds the relationship between the risk factors and mortality. Excluding such subjects purportedly reduces bias due to this confounding. The purpose of this study was to test the effects of excluding subjects who die during the first k years of follow-up on the reduction of bias under a variety of situations. METHODS: Using body mass index (BMI; kg/m2) as an example, we conducted Monte Carlo simulations to investigate such effects. RESULTS: Results suggest that under the conditions investigated, the method of excluding early deaths does not reliably or substantially reduce bias due to confounding introduced by occult disease. CONCLUSION: Excluding subjects dying during the first k years of follow-up may not be a judicious strategy for handling confounding due to occult disease. Investigators are encouraged to develop alternative methods.
PURPOSE: Estimating the effects of continuous chronic disease risk factors on mortality is an area that generates confusion and controversy. The frequently observed U-shaped or J-shaped relationships between the risk factors and mortality are often in contrast with presumed monotone relationships. Therefore, some investigators suggest that subjects dying during the first k years of follow-up (where k is some positive number less than the total length of follow-up) be excluded from statistical analyses. The rationale for this approach is that subjects dying during the first k years of follow-up are likely to have some pre-existing occult disease that confounds the relationship between the risk factors and mortality. Excluding such subjects purportedly reduces bias due to this confounding. The purpose of this study was to test the effects of excluding subjects who die during the first k years of follow-up on the reduction of bias under a variety of situations. METHODS: Using body mass index (BMI; kg/m2) as an example, we conducted Monte Carlo simulations to investigate such effects. RESULTS: Results suggest that under the conditions investigated, the method of excluding early deaths does not reliably or substantially reduce bias due to confounding introduced by occult disease. CONCLUSION: Excluding subjects dying during the first k years of follow-up may not be a judicious strategy for handling confounding due to occult disease. Investigators are encouraged to develop alternative methods.