Testing the role of time in affecting unidimensionality in health instruments

Effective allocation of limited resources in healthcare delivery system requires valid measurement of health. Valid measurement of health is contingent upon the validity of the health instrument used. An important property of measurement in general is "unidimensionality": Do the numbers assigned to the qualitative attribute being measured increase with increases in the attribute? Such property of a health instrument is important because it is impossible to discuss improvements in health with treatment or improvements in health at the population level without unidimensionality. In this dissertation, we evaluate unidimensionality with respect to a model of probability of item response.;In health measurement, unidimensionality has been shown to fail because mental health and physical health tends to behave independently from one another. One possibility is that mental health and physical health may reflect two distinctive health constructs at point-in-time. However, given the role of time in health, this dissertation tests the hypothesis that, unidimensionality is more plausible once time is considered in the model.;We approached our study via investigating the measurement properties of three widely used health instruments (the EQ-5D, the SF-12v2(TM) and the SF-36RTM) using unidimensional mathematical measurement models, i.e. the Rasch models. We tested our hypothesis using both the U.S. national survey data (the Medical Expenditure Panel Survey, MEPS) and the U.S. community survey data (the Beaver Dam Health Outcomes Study, BDHOS). We examined the goodness-of-fit of the items to see whether or not mental health and physical health items fit the unidimensional model as a single scale. We also examined the gender- and disease-related differential item functioning (DIF) to see if respondents with the same level of latent health endorsed item with systematic differences due to gender and/or health condition differences.;Based on our findings, mental health items consistently misfit the unidimensional model at point-in-time measures in all three health instruments and across all disease groups. Including time as a parameter in the model improved the overall model fit in the two instruments from the U.S. national survey (the EQ-5D and the SF-12v2(TM)), but not in the SF-36RTM from the U.S. community survey. We attribute the inconsistent findings to (1) cohort differences between samples extracted from a U.S. national survey and samples from Beaver Dam eye study in Beaver Dam, Wisconsin; (2) data metric differences as there was a substantially longer follow-up (10 years) in the Beaver Dam data compared with the shorter follow-up (2 years) in the MEPS and; (3) the missing data problem for a longitudinal measurement model. Hence, further investigation is warranted given the limited empirical evidence on robustness of the Rasch model dealing with missing data in longitudinal settings.;Our findings suggest that unidimensionality requirement in a health instrument is supported when time is considered in the model. Mental health and physical health tend to form different scales in point-in-time measures but, they may be linked through time. Therefore, parameterizing time improve the overall model fit. It no longer makes sense to interpret health measure cross-sectionally. Intertemporal health context is important in health measurement.