 Relevance- Strokes in patients with AF who are not prescribed anticoagulant therapy are an important source of morbidity, mortality, and health care cost. In a randomized controlled trial, the investigators previously demonstrated that alert-based CDS reduced the frequency of symptomatic VTE in high-risk hospitalized patients not ordered for thromboprophylaxis by 41. The investigators will now conduct a randomized controlled trial to evaluate the potential impact of an alert-based CDS strategy to prevent stroke in at-risk patients with nonvalvular AF or atrial flutter not prescribed anticoagulants. The number of stroke endpoints will be too small to show a statistical difference. These data will provide information useful to assess the feasibility and calculate sample size estimates for a larger pivotal trial with clinical end points. Though the investigators research will be conducted on hospitalized patients with AF or atrial flutter, the investigators methodology can be extended to the outpatient setting in future studies. 