@article {30,
	title = {Calibration of an item pool for assessing the burden of headaches: an application of item response theory to the Headache Impact Test (HIT)},
	journal = {Quality of Life Research},
	volume = {12},
	number = {8},
	year = {2003},
	note = {0962-9343Journal Article},
	pages = {913-933},
	abstract = {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. CONCLUSION: We have established a headache impact item pool that can be used in CAT of headache impact.},
	keywords = {*Cost of Illness, *Decision Support Techniques, *Sickness Impact Profile, Adolescent, Adult, Aged, Comparative Study, Disability Evaluation, Factor Analysis, Statistical, Headache/*psychology, Health Surveys, Human, Longitudinal Studies, Middle Aged, Migraine/psychology, Models, Psychological, Psychometrics/*methods, Quality of Life/*psychology, Software, Support, Non-U.S. Gov{\textquoteright}t},
	author = {Bjorner, J. B. and Kosinski, M. and Ware, J. E., Jr.}
}
@article {31,
	title = {The feasibility of applying item response theory to measures of migraine impact: a re-analysis of three clinical studies},
	journal = {Quality of Life Research},
	volume = {12},
	number = {8},
	year = {2003},
	note = {0962-9343Journal Article},
	pages = {887-902},
	abstract = {BACKGROUND: Item response theory (IRT) is a powerful framework for analyzing multiitem scales and is central to the implementation of computerized adaptive testing. OBJECTIVES: To explain the use of IRT to examine measurement properties and to apply IRT to a questionnaire for measuring migraine impact--the Migraine Specific Questionnaire (MSQ). METHODS: Data from three clinical studies that employed the MSQ-version 1 were analyzed by confirmatory factor analysis for categorical data and by IRT modeling. RESULTS: Confirmatory factor analyses showed very high correlations between the factors hypothesized by the original test constructions. Further, high item loadings on one common factor suggest that migraine impact may be adequately assessed by only one score. IRT analyses of the MSQ were feasible and provided several suggestions as to how to improve the items and in particular the response choices. Out of 15 items, 13 showed adequate fit to the IRT model. In general, IRT scores were strongly associated with the scores proposed by the original test developers and with the total item sum score. Analysis of response consistency showed that more than 90\% of the patients answered consistently according to a unidimensional IRT model. For the remaining patients, scores on the dimension of emotional function were less strongly related to the overall IRT scores that mainly reflected role limitations. Such response patterns can be detected easily using response consistency indices. Analysis of test precision across score levels revealed that the MSQ was most precise at one standard deviation worse than the mean impact level for migraine patients that are not in treatment. Thus, gains in test precision can be achieved by developing items aimed at less severe levels of migraine impact. CONCLUSIONS: IRT proved useful for analyzing the MSQ. The approach warrants further testing in a more comprehensive item pool for headache impact that would enable computerized adaptive testing.},
	keywords = {*Sickness Impact Profile, Adolescent, Adult, Aged, Comparative Study, Cost of Illness, Factor Analysis, Statistical, Feasibility Studies, Female, Human, Male, Middle Aged, Migraine/*psychology, Models, Psychological, Psychometrics/instrumentation/*methods, Quality of Life/*psychology, Questionnaires, Support, Non-U.S. Gov{\textquoteright}t},
	author = {Bjorner, J. B. and Kosinski, M. and Ware, J. E., Jr.}
}
