League tables of breast cancer screening units: worst-case and best-case scenario ratings helped in exposing real differences between performance ratings.

OBJECTIVES: Data on the performance of health boards, hospitals and medical specialists, etc., are being collected at various levels in the health-care system and are often presented as league tables. These tables ignore natural variation and/or confounders, and this introduces uncertainty about their interpretation. The purpose of this study was to devise and illustrate a method to expose the real difference between the ratings in league tables. METHODS: Two values per rating were added to the league tables: the best-case scenario and the worst-case scenario. True performance will lie somewhere between these two values. The method is illustrated using data from the Dutch breast cancer screening programme. Results : By focusing on one performance indicator and one confounder, it was possible to show shifts in the rating order of breast cancer screening units and thus expose the uncertainty about the true performance of each screening unit. CONCLUSIONS: The worst-case and best-case scenario ratings demonstrated the uncertainty within the ratings of a league table. League tables should therefore only be used with great caution and after providing the public with sufficient information