CHADS2 Score

0 = 1.9% stroke risk (95% CI 1.2–3)
1 = 2.8% stroke risk (95% CI 2–3.8)
2 = 4% stroke risk (95% CI 3.1–5.1)
3 = 5.9% stroke risk (95% CI 4.6–7.3)
4 = 8.5% stroke risk (95% CI 6.3–11.1)
5 = 12.5% stroke risk (95% CI 8.2–17.5)
6 = 18.2% stroke risk (95% CI 10.5-27.4)

References:
Gage BF. JAMA. 2001 Jun 13;285(22):2864-70. -> PubMed