One of my daughter's friends suggested that sequels would, on average, recieve lower scores than the original movies - as, at least in her experience, they were invariably worse. I thought I'd just confirm her suspicions so that I could let her know that she was thinking about the problem in a good way.
However, to my surprise the opposite appears to be true. Here is the mean score - the 0.5879992 number (adjusted for various things) for each episode of Sex in the City.
Sex and the City: Season 1 0.5879992 41138
Sex and the City: Season 2 0.5824835 43795
Sex and the City: Season 3 0.6523933 38983
Sex and the City: Season 4 0.7066851 34616
Sex and the City: Season 5 0.7359862 33380
Sex and the City: Season 6: Part 1 0.8097552 33532
Sex and the City: Season 6: Part 2 0.8241694 27914
As you can see the later the sequel the better the result. This seems, at least to me, counter intuitive - However the answer may lie in the second number which is the number of people who rated the movie. It seems that once there have been (in this case two episodes), then people who don't like the movie drop out and don't watch any further. So although less people watch it, they give a higher average rating.
This might be interpreted as some form of signalling. If a movie can accurately 'signal' to its potential audience that it is worth watching then the average rating will be higher. Interestingly this is - potentially - in conflict with the aim of the movie companies who might want to maximise the number of people watching irrespective of what they think of the movie (at least in the short term).