Letters

Casemix factors may not have been considered sufficiently

How can the authors be sure that other casemix factors did not account for the observed differences in 90 day mortality? The favourable "process" measures reported for hospital 6 (low rates of pressure sores and wound infections and the early mobilisation of patients) may have reflected excellent care but could equally have been a result of less sick patients being admitted in the first place and this not being detected with the casemix measures used in the multivariate analysis. For instance, could social class factors (smoking history, body mass index) or other factors (drug treatment) have varied among the hospital populations and affected the results? Similarly, could the large observed differences between the sexes indicate particular comorbidities that were not measured?

The proportion of the total variability in 90 day mortality that was due to the casemix factors used in the model is not stated. If this proportion was small there may have been other casemix factors that were not taken into consideration; the modelling process used by the authors could be better judged if all the factors considered were stated and the extent of the total variability in 90 day mortality explained by the model was recorded. It seems implausible that treatment in hospital 6 should have exerted such a pronounced protective effect.

Appreciable variations in survival due to differences in the processes of care among hospitals are difficult to identify after adjustment for casemix.² Adequate adjustment for casemix is difficult, and many studies tend to collect few data with which to attempt such adjustments. This study may have considered too few casemix factors for any reliable comment to be made about the effect of the processes of care in the different hospitals.

Senior registrar in public health medicine University Department of Public Health Medicine and Epidemiology, Queen's Medical Centre, Nottingham NG7 2UH

C J Packham 

1. Todd CJ, Freeman CJ, Camilleri-Ferrante C, Palmer CR, Hyder A, Laxton CE, et al. Differences in mortality after fracture of hip: the East Anglian audit. BMJ 1995;310:904-8. (8 April.) [Abstract/Free Full Text]
2. Gulliford MC, Petruckevitch A, Burney PGJ. Survival with bladder cancer, evaluation of delay in treatment, type of surgeon, and modality of treatment. BMJ 1991;303:437-40.