Another look at aggregate changes in severe cognitive impairment: further investigation into the cumulative effects...

This study explored whether previously reported declines in severe cognitive impairment were robust to cumulative effects of potentially confounding survey design issues.

Using the 1993 Asset and Health Dynamics of the Oldest Old study (n = 7,443) and 1998 Health and Retirement Survey (HRS; n = 7,624) the proportion of persons ages 70 and older with severe cognitive impairment was calculated under various assumptions about item nonresponse, differential loss to follow-up, and the size and composition of the nursing home population. Impairment was measured for self-respondents using a modified version of the Telephone Interview Cognitive Screen; for proxy respondents ratings of memory and judgment were used. Chi-square tests were adjusted to account for complex survey designs.

Ignoring loss to follow-up, excluding nursing home residents, and assigning a low score to those refusing subscales yielded a statistically significant decline in severe cognitive impairment from 5.8% in 1993 to 3.8% in 1998, or an average annual decline of 6.9%. When cumulative effects of survey design issues were considered and design effects incorporated into statistical tests, statistically significant declines persisted, albeit at a reduced average annual rate, ranging from 2.5% to 6.9% per year.

Previously reported improvements in severe cognitive impairment appear to be robust to a variety of specifications. Replication with future waves of the HRS and other data is warranted.