Table 6.

Variance components for multi-item measures of the diary.

Source of variance	Headache impactMIDASa	% overall variance
Personb	0.060	27.91%
Dayc	0.002	0.93%
Itemd	0.017	7.91%
Person-by-daye	0.034	15.81%
Person-by-itemf	0.005	2.33%
Day-by-itemg	0.000	0.00%
Errorh	0.097	45.12%
Total	0.22	100.00%
Between Subjects Reliability (R1F)i	0.66
Between Subjects Reliability (RKF)i	0.98
Within Subjects Reliability (RC)i	0.51

^aPedMIDAS measure was not explored due to the small sample size that completed this measure.

^bPerson = variance due to between-person differences across all days and items.

^cDay = variance due to differences between days across all persons and items.

^dItem = variance due to responses to scale items across all persons and days.

^ePerson-by-day = variance due to between-person differences at different days across all items.

^fPerson-by-item = variance due to between-persons differences in responses to scale items across all days.

^gDay-by-item = variance due to differences between days in responses to scale items across all persons.

^hError = Person x Day x Item interaction plus random error (unknown sources of variance).

ⁱR_1F, R_KF, and R_C are forms of internal consistency calculated using formulas from Cranford et al (2006) [43].