We estimated Spearman correlations between the number of systematic reviews in two important databases (the Cochrane database of systematic reviews (CDSR) and the database of abstracts of reviews of effects (DARE)) and the burden of disease (globally and in established market economies) across disease categories. We also estimated the burden of disease for each available review measured in disability adjusted life years (DALYs).2,3
We used 1990 estimates of burden of disease because studies included in systematic reviews would have responded to recent past health needs. Results with estimates from 2000 were similar.
We categorised tar geted diseases in 923 reviews from the CDSR and 1899 reviews from the DARE in issue 4, 2000, of the Cochrane Library using 20 categories of the global burden of disease taxonomy.3
We excluded unclassifiable topics (health systems, pain or anaesthesia, general operative techniques, and smoking cessation). To avoid small contributors to burden of disease, a separate analysis retained only the top 10 groups of disease accounting for > 90% of the global burden of disease. Reviews in the DARE came from high profile general medical journals (173), other general journals (77), specialist journals (1532), or other reports (117). Two independent investigators did categorisations and resolved disagreements by discussion.
We looked for correlation between the number of systematic reviews and the burden of disease. Given the small number of categories, modest differences in estimated correlations between databases and subgroups should not be attributed formal statistical significance.
We categorised 866 reviews from the CDSR and 1639 reviews from the DARE (898 and 1729 disease group entries). Coverage was similar across databases except the CDSR covered maternal and perinatal conditions better. Across disease groups, global DALYs for each review varied between 0.2-33.0 million in the CDSR and 0.1-5.5 million in the DARE. Among the top 10 disease groups, nutritional deficiencies, injuries, respiratory infections, and infectious diseases were most neglected (> 2 million global DALYs for each available review in either database).
Burden of disease was modestly correlated with the number of systematic reviews in the CDSR (global r = 0.54, P = 0.014; established market economies r = 0.46, P = 0.041), the DARE (global r = 0.65, P = 0.002; established market economies r = 0.76, P < 0.001) and in subgroups of the DARE.
For the top 10 disease groups, correlations between the number of systematic reviews and the global burden of disease remained unchanged in CDSR (r = 0.52, P = 0.13), but decreased in DARE (r = 0.42, P = 0.23). The burden of disease in established market economies correlated modestly with the number of reviews in the CDSR (r = 0.56; P = 0.09); correlations in the DARE were high (overall r = 0.87, P < 0.001, range 0.63-0.94 across subgroups of reviews).
The number of reviews in the DARE seemed less responsive to global burden of disease than to the burden in established market economies, but the difference was not significant. The CDSR did not show this ().
Figure 1 Number of systematic reviews in the Cochrane database of systematic reviews (CDSR) and in the database of abstracts of reviews of effect (DARE) according to the burden of disease for each major disease group; see bmj.com for disease categories