PROJECT SUMMARY – PROJECT 3 Clinicians treating patients in community settings face significant challenges;; evidence can be lacking for the patients they typically see, particularly for frail elderly people who are often excluded from randomized trials, and community hospitals may not necessarily provide the same level of expertise as the large academic medical centers where trials often take place. To address concerns about the potential for poor quality care, many institutions are using risk-adjusted performance measures, but there is considerable uncertainty about whether they can be interpreted in a meaningful way given the potential for unmeasured confounding and consequent biases. In this project, we use a combination of trial data, registry data, and claims data augmented with natural-experiments, to address these concerns along three dimensions. First, we harness the strength of a large registry, the Vascular Quality Initiative for patients with lower-extremity peripheral vascular disease, to estimate the extent of heterogeneity across patients, and hospitals, in treatment effects;; we plan to validate our estimates when an ongoing clinical trial (BEST-CLI) is completed. Second, we use the entry and exit of hospitals and physicians, and the consequent impact on population-level health, to assess the validity of our provider performance measures derived from claims. Finally, we use the natural experiments of provider entry and exit to understand whether these sudden changes are more burdensome for vulnerable populations such as minority patients and those with a diagnosis of Alzheimer’s Disease and related dementia (ADRD). We build on key strengths of the NIA P01, including extensive Medicare claims data, access to clinical registries, economic modeling, and most importantly detailed knowledge of the clinical setting through collaboration between clinicians and social scientists. This project will contribute to other projects (e.g., 4 and 2), and has the potential to substantially improve patient outcomes by improving guidelines for patient and hospital choice.