The Dispersion Measure of Mortality is a valuable tool for charting global progress in reducing inequality
Article: Shibuya & Boerma. 2005;83:162
21 March 2005 - We were delighted to see that Shibuya and Boerma (1) appreciated the importance of our work showing that global mortality convergence has been replaced by divergence since the late 1980s (2). However, we feel that some of the points raised by them deserve a response as in our view they cast doubt on aspects of our work that we believe are not justified - not least because some of them were dealt with directly by us in the paper.
As our paper states, as the editorial reiterates, and the paper by Mathers et al (3) in the same issue of the Bulletin makes abundantly clear, our knowledge about mortality in most of Africa, and many other low and middle income countries, is very poor. This situation needs addressing with great urgency. However, in the meantime, the UN mortality estimates provide the only long-term time series of infant mortality and life expectancy at birth data for all countries. These are the data we used while making no attempt to hide their limitations. As we said in our paper, these same data are widely used, including in the Human Development Indices, and the validity of all such applications are subject to the same concerns about data quality.
The editorial suggests that the standard UN approach to estimating mortality is becoming increasingly problematic in countries with inadequate mortality data (4). However, if we calculate the Dispersion Measure of Mortality (DMM) using the most recent WHO life expectancy at birth estimates for 2000-2002 (5) rather than those of the UN, the results are consistent with those we published.
We feel it would be useful to respond to some of the specific criticisms made of the DMM measure. Firstly, the editorial suggests that it is a weakness of the DMM that it is a summary measure that in itself provides no information about where the variation in life expectancy is occurring. However, this is a criticism that could be applied to other measures such as life expectancy itself, which is after all also a summary measure of the average level of mortality in a population. The value of the DMM is that trends in it indicate how the global distribution of mortality is changing. Our findings of a switch from global convergence of mortality to divergence are immensely worrying, something that the editorial appears to pass over without much comment.
Secondly, the editorial suggests that summary measures such as the DMM should be disaggregated into levels and causes of child and adult mortality, if light is to be thrown on the causes of changes in the measure. We agree with this, although investigating age and cause of death influences would be very problematic because of the severe data limitations for many countries. However, it is possible to decompose changes in the DMM into specific country influences. We have done this but did not find it particularly informative over and above what is evident from simple inspection of the data: that is the clear role of countries of sub-Saharan Africa and the former Soviet Union.
Finally, we completely agree that it is important to look at the trends in both mortality level and inequality between and within countries. To reiterate what we stated in our paper: ‘Future global progress should be judged not only in terms of whether overall life expectancy continues to improve but also according to whether mortality convergence can be re-established and accelerated’.
In conclusion we believe that the Dispersion Measure of Mortality is a valuable addition to the various tools that organisations such as the WHO should use on a routine basis in order to provide the fullest picture about the extent to which the world is indeed becoming a healthier and more equal place.
Kath Moser (1) Vladimir Shkolnikov (2) & David A Leon (3).
- Shibuya K, & Boerma JT. Measuring progress towards reducing health inequalities. Bulletin of the World Health Organization 2005;83:162.
- Moser K, Shkolnikov V, Leon DA. World mortality 1950-2000: divergence replaces convergence from the late 1980s. Bulletin of the World Health Organisation 2005;83:202-9.
- Mathers CD, Ma Fat D, Inoue M, Rao C, Lopez AD. Counting the dead and what they died from: an assessment of the global status of cause of death data. Bulletin of the World Health Organisation 2005;83:171-7.
- Murray CJL, Ferguson BD, Lopez AD, Guillot M, Salomon JA, Ahmad O. Modified logit life table system: principles, empirical validation and application. Population Studies 2003;57:165-82.
(1) Lecturer, Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, England (email: email@example.com).
(2) Head of Data Laboratory, Max Planck Institute for Demographic Research, Rostock, Germany.
(3) Professor, Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, England.