To work with numerical scales of the rating variable (i.e., the difference between c = −1 and c = 1 should be larger than that of c = −1 and c = 0), Hofmann (2004) used also a Gaussian distribution for P (c|az) in collaborative filtering.
we do not employ a Gaussian, because in our dataset, the number of rating classes is only 3, which is so small that a Gaussian distribution cannot be a good approximation of the actual probability density function.