Defining Evaluation Metrics

Introduction

Two main categories are used to decide on the quality of collaborative filtering algorithms:

The most widely used evaluation metrics in CF are the Mean Absolute Error (MAE), and the Root Mean Squared Error (RMSE).
The equations are as follows:

MAE and RMSE
Where n is the total number of ratings, pi,j is the predicted rating for user i on item j, and rij is the actual rating.

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