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Table 1 Model overview

From: Mapping of the Insomnia Severity Index and other sleep measures to EuroQol EQ-5D health state utilities

Dependent Variable (For Models I-IV) Independent Variables and Models
  Model I: ISI 7 items
U t i l i t y mod e l I = 1 - exp ( α + j = 0 7 X i β i j ) Where j = 0 0 X i β i 0 0 and, i = 1,...,7 representing 7 ISI items, j = 0,...,4, representing 5 levels of the Likert scale.
  Model II: ISI summary scores (0-28)
U t i l i t y M o d e l I I = 1 - exp ( α + X β ) Where X is the ISI summary scores range from 0(min) to 28 (max) and treated as a continuous variable in the model
EQ-5D disutility values
D i s u t i l i t y = exp ( α + X β ) Utility = 1-Disutility
Model III: ISI 4 clinical categories
U t i l i t y M o d e l I I I = 1 - exp ( α + j = 1 4 X β i j ) Where j = 1 1 X i β i 1 0 and, j = 1,...,4 representing 4 clinical classifications based on insomnia summary scores.
  Model IV: Insomnia symptoms and demographics
U t i l i t y M o d e l I V = 1 - exp ( α + i = 1 8 X i β i + X d u r a t i o n 2 2 β d u r a t i o n 2 ) Where i = 1,...,8 representing 5 symptoms, 2 demographic variables and 1 variable indicating the presence/absence of comorbidity. The 5 symptom variables were: sleep duration, quality, latency, next day sleepiness, and number of wakeup times during the night. The 2 demographic variables were age and gender. Within the model, a sleep duration squared term was an additional predictor adjusting the concave function of sleep duration on health state utilities.