Other approach for non binary ratings is to use the true Bayesian estimate, which uses all the platform ratings as the prior probability. This is what IMBD uses in its Top 250:
"The following formula is used to calculate the Top Rated 250 titles. This formula provides a true 'Bayesian estimate', which takes into account the number of votes each title has received, minimum votes required to be on the list, and the mean vote for all titles:
weighted rating (WR) = (v ÷ (v+m)) × R + (m ÷ (v+m)) × C
Where:
R = average for the movie (mean) = (Rating)
v = number of votes for the movie = (votes)
m = minimum votes required to be listed in the Top 250
C = the mean vote across the whole report"
"The following formula is used to calculate the Top Rated 250 titles. This formula provides a true 'Bayesian estimate', which takes into account the number of votes each title has received, minimum votes required to be on the list, and the mean vote for all titles:
weighted rating (WR) = (v ÷ (v+m)) × R + (m ÷ (v+m)) × C
Where:
R = average for the movie (mean) = (Rating) v = number of votes for the movie = (votes) m = minimum votes required to be listed in the Top 250 C = the mean vote across the whole report"
http://www.imdb.com/help/show_leaf?votestopfaq&pf_rd_m=A2FGE...