League tables are unreasonably simple

EDITOR—Not comparing like with like is the easy and traditional battle cry of those seeking to cast doubt on league tables of health service providers. It seems unfortunate therefore that the tables published for the benefit of the public in the Times as the hospital consultants' guide fall at the first hurdle on what seems to be a technical misuse, based on misleading comparisons, of one of the key statistics. 1 2

Ranking in the league tables is based on both standardised mortality ratios and death rates per 100 000, although these summarise some complex statistical workings.3 Standardised mortality ratios are a seemingly well understood means of comparing the mortality of a local population with that of a wider population, taking into account the age and sex distribution. But the Times supplement misleadingly refers to a standardised figure for mortality ratios of 100 as the national average, a higher figure indicating a higher than average number of deaths. Although this statement might be broadly true, it is also likely to produce biased tables as it misrepresents standardised mortality ratios and misuses this statistic.

Comparing standardised mortality ratios seems intuitive and looks reasonable until one unpicks their construction. A standardised mortality ratio uses the exposed group as the standard, meaning that the wider or national group is not the standard, which is probably where the misperception occurs. Therefore, comparisons of standardised mortality ratios with one another are invalid unless the age and sex distributions of the populations concerned are similar. The extent of the bias in making these comparisons may be small unless there are reasonably large departures from this point, but we do not know how much this departure for any one population differs from another and contributes towards its position in the table.3

This point has been …