Life expectancy estimation with breast cancer: Bias of the declining exponential function and an alternative to its use

Background. Life expectancy gain (LEG) is an outcome measure commonly estim ated with a declining exponential function in a Markov model. The accuracy of such estimates has not been objectively evaluated. Purpose. To compare L EGs from declining exponential function estimates with those calculated fro m population data, using published screening mammography studies as example s. Method. SEER-based population data are used to compare LEG calculation w ith declining exponential function estimation and empiric population data i n a new model, the "nested" Markov. Results. Analyses of the LEG of mammogr aphic screening based on the declining exponential function significantly o verestimate LEGs for younger women and underestimate them for older women. Because of offsetting errors, all-age analyses paradoxically appear accurat e. Conclusion. Declining exponential function estimates of LEGs for chronic diseases with low mortality rates and long time horizons are liable to sig nificant bias, especially with limited age cohorts.