An 83-year-old man with a history of hypertension and diabetes is referred to you for management after presenting to the emergency department last week with several hours of recent-onset palpitations. He was found to be in atrial fibrillation with a rapid ventricular rate and underwent successful electrical cardioversion. He was discharged to home from the ED with prescriptions for metoprolol and a limited supply of low-molecular-weight heparin to use while he began warfarin therapy. His point-of-care INR in the office today is 2.2 on warfarin 5 mg daily.The patient is questioning his need for anticoagulation, citing the fact that he is now back in normal rhythm after cardioversion and is not in atrial fibrillation permanently. He reports that his 50-year-old nephew has a diagnosis of atrial flutter and takes aspirin only for stroke prevention. He considers himself healthier than the average 83-year-old man and feels that his risk for stroke must be relatively low. He would like to know more about atrial fibrillation and his risk for stroke. You should tell him which of the following?