In the issue of 11 October 1997 (p 942) we published a correction to this article by Trisha Greenhalgh (30 August, p 540-3). The definition of the negative likelihood ratio was misleading in both the original article and the correction. The negative likelihood ratio is defined by the equation (1-sensitivity)/specificity, as stated in the original article, and addresses the question, “How much more or less likely is a negative result to be found in a person with, as opposed to without, the condition?”

In addition, an author's error occurred in the box on p 542 in the same article; the fraction for the negative predictive value should have been 966/987=97.8% [not 966/973=97.8%].