Slope One
Slope One is a simple and efficient type of recommender system. Introduced by Daniel Lemire and Anna Maclachlan in 2005, it involves a simpler idea than the majority of other collaborative filtering implementations. While these usually calculate the similarity between vectors of items using the cosine or the Pearson methods, the Slope One approach recommends items to users based on the average difference in preferences of items. The main idea of the algorithm is to create a linear relation between items preferences such as the relation F(x) = x + b. The name "Slope One" cames from the fact that here the "x" is multiplied by "1". It basically calculates the difference between the ratings of items for each user (for every item the user has rated). Then, it creates and average difference (diff) for every pair of items. To make a prediction of the Item A for an User 1 for example, it would get the ratings that User 1 has given to other items and a...