Saturday, September 22, 2012

Evolution of movie ratings

We've all checked a movie's IMDB rating to decide whether to go see it at the movies, but ever notice how the rating seems to drop when you check a few days later?

We here look at the evolution of 18 movie's ratings before/after their release to see whether this holds on this very small subset.

The hypothesis would be that there is self-selection bias in a movie's ratings, with die hard fans being the first to go see it (most often before its official release) and rate it, but because of their die hardness rating it higher than the average audience member.

For the 10 movies of interest I extracted (almost) daily ratings. Then, for each movie I compare the daily ratings to the rating on day 0 as baseline. This is defined as "IMDB rating delta" in the following graph:

Despite the small sample size, we can clearly see a decreasing trend starting a few days before release and continuing downwards two weeks after the official release. On average, movies, lose 0.4 out of 10 in IMDB rating over that period.

After the second weeks, things start to stabilize somewhat, as the number of ratings increases and we tend towards the movie's final long term rating. Based on the plot it seems as if there is a slight increase from week 2 onwards, but this is most likely due to the small sample size which gets even smaller in the future (while I have day 0 data for all movies, I do not necessarily have day 30 data for all).

But what about the metascore (aggregated score from based on "official" critics)? The same type of plot can be generated:

The pattern is here somewhat the same with a sharp decrease right around release date, but stabilization is much quicker. This makes sense since, as noted in the earlier post on individual movie evolutions, critics tend to publish their opinions right around release dates, not weeks after.

Hopefully I will be able to extend the analysis to more movies over greater time periods, but data extraction is in real-time unfortunately, no "back to the future" to speed things up ;-)

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