Editor—We agree with Shaw et al that the method of measuring inequality in health outcomes used in setting the government targets is problematic (A Low, UKPHA (UK Public Health Association), 13th annual public health forum, Gateshead, April 2005).1 However, the government's targets are based on reducing relative gaps in life expectancy and infant mortality.2 The slope index of inequality (SII) used by Shaw et al measures absolute gaps.
The distinction between absolute and relative gaps becomes important when comparisons are made over time. Increasing life expectancy over time with no change in the slope (constant SII) implies a faster rate of progress for those at the bottom of the scale than for those at the top. The figure shows a gain of 10 years of life expectancy for both groups; the rate of improvement for the poor group is 14% (10/70) and for the wealthy group 12.5% (10/80).
Figure 1.
Constant slope index of inequality (SII) means a greater rate of progress for the poor group
The interpretation of the SII is affected by increases or decreases in life expectancy over time. We therefore advocate the use of the relative index of inequality (RII) to compare magnitudes of inequality in health outcomes over time, as well as between different conditions and rates of service provision (A Low, UKPHA (UK Public Health Association), 13th annual public health forum, Gateshead, April 2005).3
We calculated the relative index of inequality for the data of Shaw et al by dividing the reported SII by the average life expectancy across all poverty groups. The absolute gap (SII) for both sexes from 1992-4 to 2001-3 is 4.3%, for men 3.8%, for women 4.0%. The relative gap (RII) is 2.0% for both sexes, 0.9% for men, and 2.2% for women. In this case the SII overestimates the magnitude of gap changes, because the increasing general trend is not accounted for.
In similar calculations for the other official headline target of infant mortality the SIIs would underestimate the magnitude of gap changes, as infant mortality is on a general downward trend.
Competing interests: None declared.
References
- 1.Shaw M, Davey Smith G, Dorling D. Health inequalities and New Labour: how the promises compare with real progress. BMJ 2005;330: 1016-21. (30 April.) [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Department of Health. Health inequalities—national targets on infant mortality and life expectancy—technical briefing (revised 2002). www.dh.gov.uk/assetRoot/04/07/78/96/04077896.pdf. (accessed 15 Apr 2005).
- 3.Low A, Low A. Measuring the gap: quantifying and comparing local health inequalities. J Public Health 2004;26: 388-95. [DOI] [PubMed] [Google Scholar]