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?

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