Erratum Please note that there is an error in the example line given on the specificity nomogram (figure 1 part B) in this article. The calculation lines on the nomogram are correct but the worked example given clearly does not meet the calculating lines. The correct example can be seen in the larger version of the nomogram at http://emj.bmjjournals.com/cgi/content/full/22/3/180/DC1?ck=nck . It should be noted that the error only affects the example and not the underlying nomogram itself.