How do recommendation systems work?

Question

I've always been curious as to how these systems work. For example, how do netflix or Amazon determine what recommendations to make based on past purchases and/or ratings? Are there any algorithms to read up on?

Just so there's no misperceptions here, there's no practical reason for me asking. I'm just asking out of sheer curiosity.

(Also, if there's an existing question on this topic, point me to it. "Recommendations system" is a difficult term to search for.)

Answer 1

At it's most basic, most recommendation systems work by saying one of two things.

User-based recommendations:
If User A likes Items 1,2,3,4, and 5,
And User B likes Items 1,2,3, and 4
Then User B is quite likely to also like Item 5

Item-based recommendations:
If Users who purchase item 1 are also disproportionately likely to purchase item 2
And User A purchased item 1
Then User A will probably be interested in item 2

And here's a brain dump of algorithms you ought to know:
- Set similarity (Jaccard index & Tanimoto coefficient)
- n-Dimensional Euclidean distance
- k-means algorithm
- Support Vector Machines