How does Alexa measure traffic?

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Tags

1. #combined
2. #first
3. #sites
4. #based
5. #traffic
6. #aggregated
7. #months
8. #users
9. #measure
10. #daily
11. #historical
12. #alexa
13. #toolbar
14. #computes
15. #three
16. #reach
17. #views
18. #basis
19. #millions
20. #number