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Objectives: To predict the risk of future unfavourable health outcomes in older people it is common to assess the level of both basic and instrumental activities of daily living. To accomplish this, the commonly used Katz-6 and the Lawton IADL questionnaires can be combined to form the 'Modified Katz ADL' scale, also known as the Katz-15 scale. So far, the validity and reliability of the Katz-15 scale is unknown. The objective of the current study is to investigate how well the Katz-15 is able to predict future unfavorable health outcomes and how this is related to the existing Katz-6 scale. Design: We performed a follow-up study using data from a group of 60 year and older participants from a large Dutch clinical trial. Participants: We included 2321 participants in the analysis. The average age of the study population was 74 years and 44% was male. Measurements: We studied the relation between the Katz-15 scale and a number of unfavourable health outcomes, such as hospitalization, admission to a nursing home, admission to a home for the aged and death within one year of follow-up. Results: We found the Katz-15 to be both internally consistent and strongly associated with quality of life measures. We observed moderate to strong associations between the Katz-15 and the unfavourable health outcomes All associations studied were stronger for the Katz-15 scale as compared to the Katz-6 scale. Conclusion: The results of our study indicate that the Katz-15 scale is able to reliably and validly predict future unfavorable health outcomes. This makes the scale a valuable measure in determining both basic and instrumental activities of daily living [ABSTRACT FROM AUTHOR]