Estimates of active and disabled life expectancy based on different assessment intervals

Although disability in activities of daily living (ADLs) is a highly dynamic process, analytic strategies for estimating active and disabled life expectancy have assumed stability in ADL function between periodic surveys spanning 12--24 months or have used interval estimation or instantaneous rates based on long assessment intervals. We performed a prospective cohort study to compare estimates of active and disabled life expectancy based on traditional assessment intervals of 1--2 years with those based on more frequent assessments at 1-month intervals.

Participants included 754 initially nondisabled community-dwelling persons, aged 70 years or older, who were interviewed monthly for 4 years to ascertain ADL disability. Estimates of active and disabled life expectancy were calculated using an increment-decrement life table for assessment intervals of 1 month, 1 year, and 2 years.

For each of five age groups, the monthly assessment strategy yielded the highest values for active life expectancy and the lowest values for disabled life expectancy. The 95% confidence intervals for these values, however, overlapped the corresponding point estimates for the annual and biennial strategies.

Accurate estimates of active and disabled life expectancy may be obtained from epidemiologic studies that assess ADL function no more frequently than every other year.