The primary objectives of this study are to compare the risk of major bleeding and stroke/systemic embolism (SE) events among oral anticoagulant (OAC)-naïve non-valvular atrial fibrillation (NVAF) patients initiating OAC warfarin or apixaban or dabigatran or rivaroxaban treatment.

Inclusion Criteria:

1. Had 1 or more pharmacy claim for apixaban or warfarin or dabigatran or rivaroxaban during the identification period (01-Jan-2013 to 30-Sep-2015). The first OAC pharmacy claim date during the identification period will be designated as the index date
2. Patients 18 years old or older as of the index date
3. At least 1 diagnosis of atrial fibrillation prior to or on index date, identified by any medical claim
4. At least 12 months of baseline period prior to index date with continuous enrollment

Exclusion Criteria:

1. Evidence of valvular heart disease, transient atrial fibrillation, venous thromboembolism during the 12-month baseline period or on the index date
2. Evidence of pregnancy during the study period
3. Had a pharmacy claim for warfarin, apixaban, dabigatran, rivaroxaban, or edoxaban during the 12-month baseline period
4. Had more than 1 oral anticoagulant claim on the index date

NVAF patients who were initiators of warfarin or apixaban or dabigatran or rivaroxaban treatment from 01-Jan-2013 through 30-Sep-2015. All patients who have at least 1 year of baseline data available and no prior OAC use will be included in this study.