Methodological issues in cognitive assessment and their impact on outcome measurement.

Assessment of cognitive impairment in chronic care populations is complicated by several factors that may interfere with the assessment process: physical frailty and disability; comorbid conditions such as depression; and decrements in vision, hearing, speech, and general communication. Moreover, cognitive impairment itself affects assessment of outcome domains such as depression, behavior, and function, thus contributing to several sources of measurement bias. Sources of bias are discussed in the context of findings from the literature relating individual and cognitive factors to outcome measurement. Recommendations for further methodological research are provided.