Actually -- one more follow up for the zero people who will read this -- there's a simpler explanation, which other people who do these kinds of statistics should probably note.
The average rating between 1920 and 1980 fluctuated at about 7.5. Then the number of movies exploded, and the variability in quality of them exploded even more than that. Movies that were better than the 1980 average had their ratings compressed (you can only do a little better than 7.5 on IMDB). Movies that were worse were much less compressed (you can easily do much worse than a 7.5). So the average gets dragged down since there are now both more movies getting 0-4s and movies getting 8-10s.
I realized this after plotting the median, not the mean, and observing that it stays remarkably constant, and possibly even shows a little recency bias for the last 2 years.
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Actually -- one more follow up for the zero people who will read this -- there's a simpler explanation, which other people who do these kinds of statistics should probably note.
The average rating between 1920 and 1980 fluctuated at about 7.5. Then the number of movies exploded, and the variability in quality of them exploded even more than that. Movies that were better than the 1980 average had their ratings compressed (you can only do a little better than 7.5 on IMDB). Movies that were worse were much less compressed (you can easily do much worse than a 7.5). So the average gets dragged down since there are now both more movies getting 0-4s and movies getting 8-10s.
I realized this after plotting the median, not the mean, and observing that it stays remarkably constant, and possibly even shows a little recency bias for the last 2 years.