BMJ  2005;330:1448 (18 June), doi:10.1136/bmj.330.7505.1448-b

Letter

Was Rodney Ledward a statistical outlier?

Statistical method may be difficult to apply in clinical practice

The first 150 words of the full text of this article appear below.

EDITOR—Harley et al make an interesting proposal for retrospectively detecting outliers.¹ However, we think that, as presented, the method may be quite difficult for clinical audit teams to understand and apply in routine practice.

It might be helpful to think in terms of test statistics. For each consultant in each year, the authors used the Mahalanobis distance to help calculate a kind of test statistic, and they compared this with a reference distribution. From table 2 of their paper (see bmj.com), some 16% of the consultants were flagged as outliers each year. If the most unusual 16% of consultants are flagged each year, then we would expect 3% of consultants to be flagged in three or more of five years simply by chance (simple binomial model). In practice, the authors found that 11 of about 100 consultants were flagged as outliers in three or more years, including Ledward.

. . . [Full text of this article]

Department of Community Medicine, University of Hong Kong, Hong Kong SAR, China commed@hkucc.hku.hk

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