Item response theory and health outcomes measurement in the 21st century

Item response theory (IRT) has a number of potential advantages over classi cal test theory in assessing self-reported health outcomes. IRT models yiel d invariant item and latent trait estimates (within a linear transformation ), standard errors conditional on trait level, and trait estimates anchored to item content. IRT also facilitates evaluation of differential item func tioning, inclusion of items with different response formats in the same sca le, and assessment of person fit and is ideally suited for implementing com puter adaptive testing. Finally, IRT methods can be helpful in developing b etter health outcome measures and in assessing change over time. These issu es are reviewed, along with a discussion of some of the methodological and practical challenges in applying IRT methods.