Appendix 3 – International Evidence on Disability Trends

Only two national surveys of disability have been done in New Zealand – in 1995 and 2001 respectively (Statistics New Zealand 1998, 2003). In the absence of a long time series of historical data, projection of disability prevalence was based on a comparative analysis of national disability surveys in other developed countries (as well as the New Zealand surveys). The method and results are briefly summarised below.

Method

A systematic review of the literature on trends in disability prevalence in developed countries was undertaken. The search methodology included a MEDLINE search of the literature (from which key review articles were identified) and a search of selected government health and statistical websites (from which key surveys and their results were identified). Note that different definitions of disability used in different studies mean that levels cannot be compared across countries, but trends can.

Criteria

Criteria for selection of studies were:

• Cohort study or serial cross sectional prevalence survey
• National in coverage
• Institutionalised population included (that is, those living in a residential facility that is not community based)
• Long duration, including at least five years in 1990s
• Outcomes measures able to be translated into a scale of disability rates, with ADL (severe) and IADL (moderate) limitations
• Minimal changes in data collection and analysis methods
• High participation rate / low rate of loss to follow up (cohort studies only)
• Minimal use of proxy responses
• English speaking country

Not all studies included met all criteria. Specifically, the Framingham Heart Study was included despite being non-national and excluding institutionalised people, because of its long follow up period and use of observational rather than self report measures of disability. On the other hand the US National Health Interview Survey (including its Supplements on Aging) was excluded as it proved difficult to express its endpoints in terms of severe (ADL) and moderate (IADL only) disability.

In all, 7 studies were included, comprising 5 serial cross sectional prevalence surveys and 2 longitudinal (cohort) studies. The latter, both of which are US studies (the National Long Term Care Study and the Framingham Heart Study) are considered more reliable and valid than the former, both because of their design and because of the extremely high standard of follow up, measurement and analysis achieved in both these two cohort studies.

*major method drift from earlier waves
**method drift from earlier waves rectified post hoc (Davis et al 2001)

Analysis

A standard template was developed and used to extract key data from the published results of the selected studies. Initially study results were converted into estimates of prevalence of moderate (IADL only) and severe (ADL) disability, if required. This was done for each ten year age group included in the study (there was variable age restriction across studies) and both genders.

A summary table was then prepared, showing age standardised prevalence of moderate and severe disability for the total adult population of each country (standardised to the WHO World population), with genders pooled. We pool genders because the underlying dynamic in disability prevalence should not be gender specific at least in terms of direction of change. The only exception to this is NZ, as indicated.

Finally we converted the change between extreme periods for each study to an average annual percentage change, assuming exponential behaviour.