Lies, damned lies and mortality statistics?

Chronic obstructive pulmonary disease (COPD) was the sixth leading cause of death worldwide in 1990 and will rise to the third leading cause by 2020.^1,2 These findings from the Global Burden of Disease Study form the starting point for a wide range of recent articles about COPD. However, when trying to interpret mortality statistics and other routine sources of data such as hospital admissions, it can be hard to avoid thinking of the quotation “lies, damned lies, and statistics”, attributed variably to Mark Twain or Benjamin Disraeli. In this issue of Thorax, data presented by Fuhrman et al³ show that COPD mortality in France in 2000–2 fell by around 40% compared with 1998–9 following a change from International Classification of Diseases coding version 9 (ICD-9) to ICD-10. In contrast, an increase in mortality of around 10% was seen when moving from ICD-9 to ICD-10 coding in the USA in 1999,⁴ while bridge coding exercises in England and Wales for 1999 (coding deaths to both ICD-9 and ICD-10 to look at the impact of the change) suggested an artefactual 3% increase.⁵ COPD mortality rates in France in 1999 were well under half those in the USA or UK, which may in part relate to differences in diagnostic fashions (such as continued use of “bronchitis” rather than a COPD specific term in France).³ Two important questions arise from this:

• Can clinicians have confidence in using and interpreting routine mortality data?

• Is it really possible to use routine mortality data to make international comparisons?

The answer to both is a qualified yes.

COLLECTION AND ANALYSIS OF COPD MORTALITY DATA

It is axiomatic that mortality, hospital, and other routinely collected data need careful collection, presentation and interpretation,⁶ but it may be …