Response to Hays et al and McHorney and Cohen: A Discussion of Item Response Theory and Its Applications in Health Status Assessment

*From the Center on Outcomes Research and Education (CORE), Evanston Northwestern Healthcare; the Department of Psychiatry and Behavioral Science, Northwestern University Medical School; and the Institute for Health Services Research and Policy Studies, Northwestern University, Evanston, Illinois. †From the Center on Outcomes Research and Education (CORE), Evanston Northwestern Healthcare and the Institute for Health Services Research and Policy Studies, Northwestern University, Evanston, Illinois. Supported in part by DHHS NIH grant R01 CA60068. Address correspondence and reprint requests to: David Cella, PhD, Center on Outcomes Research and Education (CORE), Evanston Northwestern Healthcare, 1000 Central St, Suite 101, Evanston, IL 60201. E-Mail: [email protected]