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Tuesday, 17 July 2018

Collaborative Filtering in Machine Learning

Recommender System :-
  Many of you have used e-commerce science like amazon flip cart and other site and main have seen recommender system working so, when you login to those system. The system recommender sudden items to you. The Recommender system based looks at the past behavior of the user and the other data that is has and prize to recommender items to the user.

There are two types of Recommender System
1. Item Recommendation
2. Rating Prediction

Collaborative Filtering for Rating Prediction

User-based Nearest Neighbor
- Neighbor = similar users
- Generate a prediction for an item i by analyzing ratings for i from users in u's neighborhood.

Neighborhood formation phase

Let the record (or profile) of the target user be u (represented as a vector), and the record of another user be v ( v ∈ T).  

The similarity between the target user, u, and a neighbor, v, can be calculated using the Pearson's correlation coefficient:


Recommendation Phase

Use the following formula to compute the rating prediction of item i for target user u
where V is the set of k similar users, rv,i is the rating of user v given to item i,
 
Issue with the user-based kNN CF

The problem with the user-based formulation of collaborative filtering is the lack of scalability:
 - it requires the real-time comparison of the target user to all user records in order to generate predictions.
A variation of this approach that remedies this problem is called item-based CF

Item-based CF

The item-based approach works by comparing items based on their pattern of ratings across users. The similarity of items i and j is computed as follows :

 Recommendation phase
- After computing the similarity between items we select a set of k most similar items to the target item and generate a predicted value of user u's rating
              where j is the set of k similar items.

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