Hi I'm fresh in this field. I'm confused about the method of accuracy in a recommend algorithm. As I know, the rating matrix of users-items is sparse. I know I can use some algorithm to predict the missing values and the model like RMSE or MAE, but how to calculate the accuracy?

For some items, I don't know their real ratings. If I use 0, it would increase the error, isn't it?

Thanks for help!