We were supplied with a dataset by the NHS Information Centre for the purpose of statistical modelling of the new SHMI indicator, including the impact of case-mix adjustment variables and the variability of the measure over time.
This study explores mortality in-hospital and up to 30 days post-discharge over a five year period to determine whether there have been improvements in case-mix adjusted mortality, to examine if any changes are due to changes in case-mix adjustment variables such as age, sex, method of admission and comorbidity, and to compare changes between hospital trusts.
Using the SHMI we have therefore sought to validate the Dr Foster finding, investigated if any changes in the national SHMI are due to changes in case-mix adjustment variables such as age, sex, method of admission and comorbidity, and compared changes between hospital trusts.
As the Dr Foster Unit method, like other case mix adjustment methods, does not report screening variables for non-constant risk, 1 12 we investigated seven variables and found that three of them age, sex, and deprivation were safe in this respect.
In addition, given the widespread use of case mix adjusted outcome comparisons in health care (for example, for producing standardised mortality ratios to compare intensive care units 8), we urge that all case mix adjustment methods should screen (and report) variables for their susceptibility to the constant risk fallacy.
Coding errors can occur in variables that are used to measure both; however case mix adjustment is widely done because adjusted comparisons are generally considered to be less biased than unadjusted comparisons [ 26].
We also computed odds ratios (ORs) and 95% CIs for the association between pollutant levels and LBW and IUGR using appropriate logistic regression models that included the same adjustment variables as in the mixed-effects models.
We also performed comprehensive case mix adjustment for multiple factors, including demographics, comorbidities, and laboratory variables thereby accounting for patient related factors that may impact dialysis access.
Two variables (sex and deprivation) had no significant interaction with hospitals, indicating that these two variables are safe to use for case mix adjustment because they are not prone to the constant risk fallacy.
