Would have been better to use t test than Mann-Whitney U test

EDITOR—Williams et al undertook a randomised trial to evaluate whether follow up of patients with inflammatory bowel disease is better with open access than with routine appointments.1 They compared primary and secondary care resource use and costs and concluded that open access follow up saves secondary care resources. This conclusion, however, is mistaken because they used inappropriate statistical methods.

Resource use and cost data tend to have highly skewed distributions. As a result, the authors decided that standard parametric statistical methods were not appropriate and assessed significance by using a Mann-Whitney U test. Although this is consistent with conventional statistical guidelines,2 it does not address the question of interest in economic evaluations. As the authors themselves state, “economic analysis is mainly concerned with a comparison of means.” Use of a Mann-Whitney U test, however, makes an overall comparison of distributions in the two groups, in terms of both shape and location,3 and does not specifically test for a difference in means.

The most appropriate simple method for comparing mean costs is the ordinary t test. By using the means and standard deviations in each group reported by the authors, we have calculated P values from t tests (table). The conclusions are dramatically different from the authors'. In …