Calibration of an item pool for assessing the burden of headaches: an application of item response theory to the headache impact test (HIT).
Bjorner JB, Kosinski M, Ware JE. Calibration of an item pool for assessing the burden of headaches: an application of item response theory to the headache impact test (HIT). Qual Life Res. 2003 Dec; 12(8):913-33.