Reliability of league tables of in vitro fertilisation clinics: retrospective analysis of live birth rates   Commentary: How robust are rankings? The implications of confidence intervals

Abstract

Objective: To determine to what extent institutions carrying out in vitro fertilisation can reasonably be ranked according to their live birth rates.Design: Retrospective analysis of prospectively collected data on live birth rate after in vitro fertilisation. Setting: 52 clinics in the United Kingdom carrying out in vitro fertilisation over the period April 1994 to March 1995. Main outcome measure: Estimated adjusted live birth rate for each clinic; their rank and its associated uncertainty. Results: There were substantial and significant differences between the live birth rates of the clinics. There was great uncertainty, however, concerning the true ranks, particularly for the smaller clinics. Only one clinic could be confidently ranked in the bottom quarter according to this measure of performance. Many centres had substantial changes in rank between years, even though their live birth rate did not change significantly. Conclusions: Even when there are substantial differences between institutions, ranks are extremely unreliable statistical summaries of performance and change in performance, particularly for smaller institutions. Any performance indicator should always be associated with a measure of sampling variability. Key messages Institutional ranks are extremely unreliable statistical summaries of performance Institutions with smaller numbers of cases may be unjustifiably penalised or credited in comparison exercises Additional statistical analysis may help to identify the few institutions worthy of review Any performance indicator should always have an associated statistical sampling variability