2.3 International evidence on disability trends
It is difficult to predict the net effect of medical progress on age-specific disability rates. Some new technologies have led to increased disability rates: the standard example is coronary care, which has reduced the case fatality of heart attack, but in so doing has created an "epidemic" of heart failure. However, other technologies such as drugs to reduce hypertension (the major risk factor for stroke) have helped reduce disability rates. Similarly, it is difficult to predict the net effect on disability of conflicting population health trends such as increasing obesity and declining smoking rates. The only way to resolve the uncertainty is to look at longitudinal data on disability.
We have carried out a systematic review of longitudinal studies from New Zealand and elsewhere. Appendix 3 provides details on the methodology of the review, and on the studies that were included. The studies reviewed came to different conclusions. For instance, comparison of the 1996 and 2001 Post-Censal Disability Surveys in New Zealand suggested that disability rates were approximately constant during that period, while Census data from Australia appeared to imply that rates had risen somewhat. The highest-quality studies, covering the longest periods, were conducted in the United States. These studies all suggested that significant declines in disability rates had been occurring.
