Researchers use the Delphi consensus method to make quantitative predictions from qualitative analysis—and yes, it is named after the ancient Greek oracle. The consensus emerges when members of a panel of experts independently review data and then anonymously share their findings. Each member of the panel then gets an opportunity to revise his or her own estimates while considering their colleagues’ predictions. Human nature being what it is, the panel tends to converge on a reasonably solid consensus without the added stress of peer pressure. These consensus methods have become widely accepted in recent years. They offer an advantage over the more quantitative meta-analyses because, being based on qualitative assessment, they can include studies that have sparse or poor data. In fact, when numbers are missing, the method allows the experts to fill in the blanks by extrapolating from similar data obtained elsewhere. One hopes that Sybil, the guardian priestess, would approve.
Ferri and colleagues trained the consensus technique on epidemiology data published between 1980 and 2004 and came up with predictions for the prevalence of dementia in 14 World Health Organization (WHO) regions. WHO defines these based on both geography and patterns of child and adult mortality. The authors stuck to dementia rather than AD or other specific dementias because most of the published data does not document the ratio of different subtypes.
The consensus panel of 12 experts estimated that the total number of people with dementia worldwide today is just over 24 million, which fits some previous predictions (see, for example, Wimo et al., 2003). But they paint a dire picture of how dementia numbers are set to increase over the next 35 years or so, especially in developing countries.
The authors predict that the number of people with dementia in developed countries, which already have a high prevalence, will double between 2001 and 2040. In developing countries, however, the numbers are set to increase by up to fourfold. China and the developing western Pacific, for example, will have more than 26 million dementia patients, aged 60 and older, in 2040. That is up from an estimated six million in 2001. Worldwide, cases are set to double every 20 years until 2040.
The authors caution that their predictions have limitations. “Although the expert panel achieved high levels of consensus, this was often on the basis of scant epidemiological evidence,” they write. Nonetheless, they also suggest that their estimates for developing countries are, if anything, conservative. As patterns of morbidity and mortality in developing countries converge with those in the developed West, dementia prevalence will do likewise, they suggest. “The implication is that our projections for dementia in developing regions might be conservative,” they write.
The bigger question here is how this data will be used. In ancient Greece, the government consulted the oracle at Delphi before making major policy decisions. One can only hope that current lawmakers will heed these latest predictions, too.—Tom Fagan.
Ferri CP, Prince M, Brayne C, Brodaty H, Fratiglioni L, Ganguli M, Hall K, Hasegawa K, Hendrie H, Huang Y, Jorm A, Mathers C, Menezes PR, Rimmer E, Scazufca M; Alzheimer's Disease International. Global prevalence of dementia: a Delphi consensus study. Lancet. 2005 Dec 17;366(9503):2112-7. Abstract