Ghali WA, Quan H, Brant R, van Melle G, Norris CM, Faris PD and Knudtson ML for the APPROACH Investigators. A comparison of two methods for calculating adjusted survival curves from proportional hazards models. JAMA 2001 Sept26;286(12):1494-97.
Context: Adjusted survival curves are often presented in medical research articles. The most commonly used method for calculating such curves is the mean of covariates method, in which average values of covariates are entered into a proportional hazards regression equation. Use of this method is widespread despite published concerns regarding the validity of resulting curves.
Objective: To compare the mean of covariates method to the less widely used corrected group prognosis method in an analysis evaluating survival in patients with and without diabetes. In the latter method, a survival curve is calculated for each level of covariates, after which an average survival curve is calculated as a weighted average of the survival curves for each level of covariates.
Design, Setting, and Patients: Analysis of cohort study data from 11,468 Alberta residents undergoing cardiac catheterization between January 1, 1995 and December 31, 1996.
Main Outcome Measures: Crude and risk-adjusted survival for up to 3 years after cardiac catheterization in patients with vs without diabetes, analyzed by the mean of covariates method vs the corrected group prognosis method.
Results: According to the mean of covariates method, adjusted survival at 1044 days was 94.1% and 94.9% for patients with and without diabetes, respectively, with misleading adjusted survival curves that fell above the unadjusted curves. With the corrected group prognosis method, the corresponding survival values were 91.3% and 92.4%, with curves that fell more appropriately between the unadjusted curves.
Conclusions: Misleading adjusted survival curves resulted from using the mean of covariates method of analysis for our data. We recommend using the corrected group prognosis method for calculating risk-adjusted curves.