Study of highest grossing Hollywood movies from 1935 to 2005.
Some analysis from the Go Into the Story Blog
I would also say that there have been many cultural taste changes over time, especially in editing style. My first thought here is still correlation and causation, and some statistical problems as in 1980 - on more films were produced AND "New Hollywood" found the formula we tend to watch today. Screenwriting and story gurus such as Joseph Campbell and Robert McKee really began to take hold in this time as well, which emphasized structural guidelines.
In other words, this mathematical analysis seems skewed directly toward already formulaic films.
That being said - that formula has been arrived at by nearly a century of trial and error, measured by box office success.