Trimming cost data
The article by Thompson and Barber addresses an important problem for the
costing of healthservice activity. It does however ignore a considerable
body of work undertaken around casemix methods, stimulated orignially by
the work of Fetter on Diagnosis Related Groups (DRGs) in the early 80's.
This casemix based work has used a trimming approach to exclude abnormally
long stay or costly patients and produce a less skewed population which
can be analysed with parametric methods and provide better comparisons.
Considerable debate and empirical work has been undertaken to find
suitable methods of trimming, these have included simple percentiles,
multiples of the standard deviation of the log distribution and multiples
of the interquartile range. The objective of this work has been to find a
technique which maximises the improvement of the remaining distribution,
whilst minimising the number of cases excluded from the analysis.
Whilst this creates a problem of how to handle the costs of the
outlier patients, it permits a better comparison of the average patients
and has therefore been widely used in the many applications of casemix for
funding and performance monitoring in the US and many other countries. The
National Schedule of Reference Costs published by the NHS Executive (
January 2000), provides costs for Healthcare Resource Groups ( the English
analogue of DRGs) , excluding the trimmed cases, and the excess ( outlier
cases) costs. Similarly, length of stay comparisons based on HRG analysis
of Hospital Episode Statistics ( as supplied by a number of benchmarking
services) also use trimmed length of stay data together with a comparison
of the proportion of episodes trimmed.
Competing interests: No competing interests