PURPOSE: The amount of immortal time bias in studies with nonfatal outcomes is unclear. To quantify the magnitude of bias from mishandling of immortal person-time in studies of nonfatal outcomes.
METHODS: We derived formulas for quantifying bias from misclassified or excluded immortal person-time in settings with nonfatal outcomes, assuming a constant rate of outcome. In the situation of misclassified or excluded immortal person-time, the quantification includes the immortal time and corresponding events mistakenly attributed to the exposed group (misclassified) or excluded from study (excluded) that must be attributed to the comparison group.
RESULTS: With misclassified immortal person-time, the magnitude of bias varies according to the incidence rate ratio of immortal time and comparison group as well as the rate ratio of immortal time and exposed group: toward null for both ratios less than 1, no bias for both ratios equal to 1, away from null for both ratios greater than 1. For one ratio less than 1 and the other greater than 1, the direction and magnitude of bias can be obtained from the formula provided. With excluded immortal person-time, the magnitude of bias is associated with the incidence rate ratio of immortal time and comparison group: toward null for the ratio less than 1, no bias for the ratio equal to 1, and away from null for the ratio greater than 1.
CONCLUSIONS: Bias due to immortal person-time in studies with nonfatal outcomes can vary widely and can be quantified under assumptions that apply to many studies.
