Reliability
Test–retest reliability was assessed in a population considered to be stable. This group received placebo and had a change of one day or less in their number of migraine-headache days per month. WPAI domain scores at baseline and Months 3 were used to assess test–retest reliability. Intra-class correlation coefficients (ICC) were calculated using the two-level random-intercept-only mixed model [The ICC values were classified as: 0.01 to 0.20 (slightly fair), 0.21 to 0.40 (fair), 0.41 to 0.60 (moderate), 0.61 to 0.80 (substantial), and 0.81 to 1.00 (almost perfect agreement) [