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. 2012 Feb 3;2(1):e000746. doi: 10.1136/bmjopen-2011-000746

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© 2012, Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.

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Distribution of x-rays with a normal diagnosis in the two populations: high prevalence (red) and low prevalence (blue). Shown are the percentage of normal x-rays in each population (high or low prevalence), which are of a particular type. For example, 10% of x-rays diagnosed normal in the high-prevalence (red) population were of elbows. Differences in the distributions between the high- and low-prevalence populations could potentially account for differences in the specificity between the respective populations. Note that the normal diagnosis refers to the reference standard diagnosis. T & L, thoracic and lumbar.