Breast and cervical cancer survival: making sense of "league tables".

Abstract

During 1998, the Department of Health proposed to use survival rates of cervical and breast cancer in the 1989/90 incidence cohort as indicators of care. Valid interpretation was of concern within Trent and the Trent Cancer Registry responded by performing additional analyses. Trent Cancer Registry registrations for 1989/90 were re-analysed and the stability of districts' ranks for that cohort was investigated using random simulation techniques. Stability of ranks across more recent cohorts was investigated and attempts made to use all available information. The Department of Health's analyses were confirmed by our re-analysis of the 1989/90 cohort: Rotherham residents appeared to have the "worst" survival for cervical cancer, and Sheffield residents for breast cancer, although not statistically significantly so. Random simulations indicated that ranks based on a single cohort are not stable: for example Sheffield (ranked tenth for 1-y breast cancer survival) was ranked third or better in 6% of randomisations. Ranks were also unstable across cohorts: for example Rotherham 1-y cervical cancer survival was ranked tenth for 1989/90, fifth for 1991/92 and tenth for 1993/94. Analysis of 3-y running averages provided better information than the league table approach. Most districts improved over time, to different degrees, and similar sized gaps remained between the "best" and the "worst" districts. This analysis illustrates the need to be circumspect when interpreting "league tables" based on a single year or cohort analysis. League tables are based on ranks: clearly a large difference in rank may reflect only trivial (ie medically unimportant) differences in actual outcome. Lack of a statistically significant difference in survival between two districts does not mean their survival is equivalent. Even for a common cancer, like breast cancer, rankings were unstable from cohort to cohort. At the Registry we propose to perform these trend analyses routinely in future, adjusting, when possible, for the effects of deprivation and stage at diagnosis.