Interpretation of health and vision utilities in low vision patients.

Purpose: The purpose of the study is to evaluate the relationship between time trade off (TTO) and standard gamble (SG) estimates of health and vision utilities in a low vision patient sample.

Methods: Telephone surveys were conducted on 74 low vision patients. All study participants were administered utility questionnaires that used the TTO and SG methods as they relate to health and vision.

Results: There is high between-person variability in the relationship of TTO- to SG-estimated utilities for both vision and health. However, when transformed to logits, differences between TTO and SG utilities for health are equal to differences between TTO and SG utilities for vision. These differences are symmetrically distributed around the origin. The data were consistent with a model that includes both health or vision state and personal response criteria. The model explains between-person variability in the relationship of TTO to SG utilities as idiosyncratic differences within people between response criteria for making TTO and SG judgments.

Conclusions: The large between-person variability in the relation of utilities estimated from TTO to those estimated from SG can be explained by large between- and within-person variability in personal TTO and SG response criteria. However, within each person, the response criteria used to judge health state are the same as the response criteria used to judge vision state. This observation leads to the conclusion that health and vision states are in the same units when estimated from utilities. A meta-analysis of published studies that compared TTO with SG utilities for different health states confirms the conclusion of the model that average utilities across people are criterion-free estimates of average health-related states on a common logit scale.