Purpose: In a nationally representative sample, we tested the validity of time tradeoff (TTO) elicitations conducted over the telephone. TTO elicitations are cognitively demanding, and some experts have questioned their validity in people with low numeracy. We attempted to test the validity of the phone administered TTO by comparing it against another utility measure, the subjective health rating scale (0-100).

Methods: We surveyed a random subset of 1031 people participating in the Health and Retirement Study, a representative sample of people in the U.S. age 50 and older. The HRS includes 3 types of health measures: the SF1, a comorbidity index, and a list of ADL limitations. It also includes several measures of cognition: a 3-item numeracy measure, a serial 7 task and a memory test. We looked at how the TTO and subjective health rating correlated with the health and ADL composites. To test if the correlation between health and TTO values was a function of cognition, we divided participants by median split on each of the cognitive measures.

Results: Rating scale scores were significantly correlated with the SF1, objective health, and ADL (r's - .295 to - .579, p's < .0001). We found no relationship between the TTO scores and either the SF1 or objective health (p's > .06), while correlation with ADLs was significant, but low (r = -.105 p<.001). Further analysis showed no relationship between TTO scores and self reported and objective health measures for subjects who scored below median on any of the cognitive measures, a finding that extends Woloshin's research. However, TTO scores and objective health were significantly correlated for participants who scored above the median on all three cognitive tasks, although correlations were low (r's -.096 to -.119, p's < .05). TTO scores were significantly correlated with the SF1 and ADL ratings only for subjects scoring above the median on the serial subtraction task (r = -.098, p < .031 and r = -.110, p < .015, respectively).

Conclusions: The validity of telephone TTO elicitations is questionable among the majority of people over the age of 50, with no correlation between health and TTO among people with below average cognitive abilities, and low and inconsistent correlations for those with above average abilities.