A psychometric analysis of the measurement level of the rating scale, time trade-off, and standard gamble.

A fundamental assumption of utility-based analyses is that patient utilities for health states can be measured on an equal-interval scale. This assumption, however, has not been widely examined. The objective of this study was to assess whether the rating scale (RS), standard gamble (SG), and time trade-off (TTO) utility elicitation methods function as equal-interval level scales. We wrote descriptions of eight prostate-cancer-related health states. In interviews with patients who had newly diagnosed, advanced prostate cancer, utilities for the health states were elicited using the RS, SG, and TTO methods. At the time of the study, 77 initial and 73 follow-up interviews had. been conducted with a consecutive sample of 77 participants. Using a Rasch model, the boundaries (Thurstone Thresholds) between four equal score sub-ranges of the raw utilities were mapped onto an equal-interval logit scale. The distance between adjacent thresholds in logit units was calculated to determine whether the raw utilities were equal-interval. None of the utility scales functioned as interval-level scales in our sample. Therefore, since interval-level estimates are assumed in utility-based analyses, doubt is raised regarding the validity of findings from previous analyses based on these scales. Our findings need to be replicated in other contexts, and the practical impact of non-interval measurement on utility-based analyses should be explored. If cost-effectiveness analyses are not found to be robust to violations of the assumption that utilities are interval, serious doubt will be cast upon findings from utility-based analyses and upon the wisdom of expending millions in research dollars on utility-based studies.

SODA (severity of dyspepsia assessment): a new effective outcome measure for dyspepsia-related health.

The aim of this research was to develop and evaluate an instrument for measuring dyspepsia-related health to serve as the primary outcome measure for randomized clinical trials. Building on our previous work we developed SODA (Severity of Dyspepsia Assessment), a multidimensional dyspepsia measure. We evaluated SODA by administering it at enrollment and seven follow-up visits to 98 patients with dyspepsia who were randomized to a 6-week course of omeprazole versus placebo and followed over 1 year. The mean age was 53 years, and six patients (6%) were women. Median Cronbach's alpha reliability estimates over the eight visits for the SODA Pain Intensity, Non-Pain Symptoms, and Satisfaction scales were 0.97, 0.90, and 0.92, respectively. The mean change scores for all three scales discriminated between patients who reported they were improved versus those who were unchanged, providing evidence of validity. The effect sizes for the Pain Intensity (.98) and Satisfaction (.87) scales were large, providing evidence for responsiveness. The effect size for the Non-Pain Symptoms scale was small (.24), indicating lower responsiveness in this study sample. SODA is a new, effective instrument for measuring dyspepsia-related health. SODA is multidimensional and responsive to clinically meaningful change with demonstrated reliability and validity.

Evaluation of a multidimensional measure of dyspepsia-related health for use in a randomized clinical trial.

In previous work, we developed a multidimensional measure of dyspepsia-related health. To evaluate the adequacy of this instrument as an outcome measure for a large-scale, multicenter, randomized clinical trial, we used Rasch analysis to address three questions: (1) Are the scales interval-level? (2) Do the scales measure precisely across the entire range of dyspepsia outcomes? (3) Do the scales' items have an optimal number of response categories? We found that the scales were not interval-level and that they did not measure effectively at low or high levels of the dyspepsia-related outcomes. Our results also suggest that patients were capable of discriminating among only four- to seven-item response categories. Further studies are needed to identify items that effectively measure high and low levels of dyspepsia-related outcomes and to validate that decreasing the number of response categories improves the psychometric properties of these scales.