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.
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.
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
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.