Intended for healthcare professionals

Rapid response to:

Education And Debate

How should cost data in pragmatic randomised trials be analysed?

BMJ 2000; 320 doi: (Published 29 April 2000) Cite this as: BMJ 2000;320:1197

Rapid Response:

Misleading arithmetics

Relying on the arithmetic mean to describe highly
skewed distributions seems counter-intuitive and
clearly misleading.

In effect, this assumes that the prevalence of factors
leading to high individual costs is fixed and/or that both
this prevalence and the estimate of the costs incurred
for these patients can be generalized. The variance of
the costs estimates will also be inflated by an unknown
factor, leading to increased Type II errors for
comparisons of alternative programs or interventions
such as reported by the authors.

If economic evaluation is intended to provide decision
makers with non biased information that can help them
assessing the costs and consequences of alternative
interventions, it might be more informative to report:

1) the actual distributions of the costs incurred for each

2) unbiased estimates of the central tendency and of
the dispersion of these distributions, i.e. medians and
quartiles or geometric means and confidence intervals
if the distribution are highly skewed, or yet arithmetic
means after excluding "outliers" . Non parametric or
bootstrap tests would also seem adequate for
assessing differences between these distributions.

3) estimates of the prevalence of outliers, and of the
costs incurred for these patients.

Sensitivity analysis methods could then (and should)
be used to assess the impact of variations in the
prevalence of "outliers" on the overall conclusions of
the analysis.

Competing interests: No competing interests

29 April 2000
Alain Fontaine
Medical Evaluation Unit
Hôpital Louis Mourier, Colombes, France