The National Institute on Aging (NIA) invites applications for short-term projects involving secondary analysis aimed at the following goals: To assess the public health and health cost impact of specific combinations of two or more conditions in defined older populations, To identify potential differences in effectiveness and safety of different treatment regimens for patients with specific combinations of two or more conditions, To examine alterations in safety or effectiveness of a treatment for one condition related to the presence of one or more specific coexisting condition. To identify and address methodological issues relevant to analyses of the health impact of multiple chronic conditions such as validity of data and confounding by indication.