Calibration of an item pool for assessing the burden of headaches: an application of item response theory to the headache impact test (HIT).
BACKGROUND
Measurement of headache impact is important in clinical trials, case detection, and the clinical monitoring of patients. Computerized adaptive testing (CAT) of headache impact has potential advantages over traditional fixed-length tests in terms of precision, relevance, real-time quality control and flexibility.


OBJECTIVE
To develop an item pool that can be used for a computerized adaptive test of headache impact.


METHODS
We analyzed responses to four well-known tests of headache impact from a population-based sample of recent headache sufferers (n = 1016). We used confirmatory factor analysis for categorical data and analyses based on item response theory (IRT).


RESULTS
In factor analyses, we found very high correlations between the factors hypothesized by the original test constructers, both within and between the original questionnaires. These results suggest that a single score of headache impact is sufficient. We established a pool of 47 items which fitted the generalized partial credit IRT model. By simulating a computerized adaptive health test we showed that an adaptive test of only five items had a very high concordance with the score based on all items and that different worst-case item selection scenarios did not lead to bias.


CONCLUSIONS
We have established a headache impact item pool that can be used in CAT of headache impact.
Bjorner
Jakob B
JB
QualityMetric Incorporated, Lincoln, RI 02895, USA. jbjorner@qualitymetric.com
Kosinski
Mark
M
Ware
John E
JE
D000293:Adolescent; D000328:Adult; D000368:Aged; D002138:Calibration; D006233:Persons with Disabilities; D005163:Factor Analysis, Statistical; D006261:Headache / Q000503:physiopathology*; D006801:Humans; D008875:Middle Aged; D011788:Quality of Life; D018434:Sickness Impact Profile; D011795:Surveys and Questionnaires