function pdf = ratingPdf(meanRatings)
 sigma = 1.0;
 ratings = [1:5]';
 knownIndices = find(meanRatings~=0);
 for i=knownIndices
   pdf(:,i) = normpdf(ratings,meanRatings(i),sigma);
   pdf(:,i) = pdf(:,i)./sum(pdf(:,i));
end
 
