Send to

Choose Destination
See comment in PubMed Commons below
Chest. 2000 Apr;117(4):1112-7.

Effect of varying the case mix on the standardized mortality ratio and W statistic: A simulation study.

Author information

Department of Anesthesiology, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA.



To evaluate the validity of using the standardized mortality ratio (SMR) and the W statistic as risk-adjusted measures of hospital mortality to judge ICU performance.


APACHE (acute physiology and chronic health evaluation) II data were collected prospectively from the surgical ICU (SICU) at a single institution using all adult admissions (n = 6806) over an 8-year period (excluding cardiac surgical patients, burn patients, and patients under 16 years of age). Using a computer simulation technique, virtual ICUs (VICUs) with mortality rates between 5% and 16% were constructed. After first dividing the original data set into deciles of risk, each VICU was constructed by randomly resampling between 10 and 680 patients from each decile. The SMR, W statistic, and Z statistic were calculated for 10,000 different case mixes.


The SICU at a 450-bed teaching hospital.


A group of 6,806 adult patient admissions, excluding cardiac surgical patients and burn patients.


VICUs were created from a data set of actual patients treated at one institution in order to test the hypothesis that the SMR and W statistic would remain invariant when applied to subsets of patients from a single institution. Instead, the SMR and W statistic were found to be very sensitive to changes in case mix. The SMR and W statistic were linear functions of the simulated ICU mortality rate.


This simulation demonstrates that the SMR and the W statistic based on APACHE II cannot be used to compare outcomes of ICUs. We have proposed a revision of the SMR that eliminates the effect of case mix and allows for more accurate comparisons of ICU performance.

[Indexed for MEDLINE]
PubMed Commons home

PubMed Commons

How to join PubMed Commons

    Supplemental Content

    Full text links

    Icon for Elsevier Science
    Loading ...
    Support Center