Differences in case-mix can influence the comparison of standardised mortality ratios even with optimal risk adjustment: an analysis of data from paediatric intensive care

Abstract

Even if two healthcare providers are performing equally for each type of patient, if their patient populations differ in case-mix their SMRs will not necessarily take the same value. Clinical teams and commissioners must always keep in mind this weakness of the SMR when making decisions.

Survival of Infants Born at Periviable Gestational Ages

“…31 This paradox has been demonstrated in studies using risk-adjustment to compare standardized mortality ratios among different populations. 32,33 …”

Simpson's paradox: how performance measurement can fail even with perfect risk adjustment

“…In this issue, Manktelow et al 12 explain how the same phenomenon may occur when comparing standardised mortality ratios (SMRs). Even though two providers have the same mortality risk within subgroups of high-risk and low-risk patients, their overall SMRs differ due to the difference in case-mix of patients that they treat.…”

“…The paper by Manktelow et al 12 and a recent paper from Pouw and colleagues24 provide empirical examples of how differences in case-mix can affect the HSMR. These examples highlight the need for caution when hospitals differ in the proportions of high-risk patients that they treat.…”

What is a performance outlier?

“…In all these examples, even with perfect patient-level risk adjustment, comparisons among providers may be biased and inaccurate unless differences in the relative distributions of higher and lower risk cases are properly accounted for,4 23 a profiling analogue of Simpson's paradox 24 25. The impact of this phenomenon is not uniform.…”