IMDb does not use the arithmetic mean when calculating the ratings, although the raw mean and median are shown under the detailed vote breakdown graph on the ratings pages. Instead we use a weighted average scheme. In order to avoid leaving the scheme open to abuse, we do not disclose the exact methods used. You may find more information on Weighted Average Ratings.
This ratings system is applied universally across the database to all movies, without exception, to ensure there is no bias. The system combines a number of well-known and proven statistical methods, including a trimmed mean to reduce extreme influences and, most importantly a complex voter weighting system to make sure that the final rating is representative of the general voting population and not subject to over influence from individuals who are not regular participants in the poll. The system has been developed internally over the 10 years the poll has been in operation and tuned on a regular basis to make sure it remains fair.
The objective of the scheme is to present a more representative rating which is immune from abuse by subsets of individuals who have combined together with the aim of influencing (either up or down) the ratings of specific movies. This includes people involved in the production of a movie and their friends or fans trying to unduly raise the rating of a movie far above that of where the typical IMDb users would rate it.
IMDb takes pride in the unbiased nature of our database. This is why we also display the raw vote totals along with the mean and median votes to provide transparency to our users.