|Dependent Variable (For Models I-IV)||Independent Variables and Models|
Model I: ISI 7 items|
Where 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)|
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|
Utility = 1-Disutility
Model III: ISI 4 clinical categories|
Where and, j = 1,...,4 representing 4 clinical classifications based on insomnia summary scores.
Model IV: Insomnia symptoms and demographics|
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.