I am analysing data about movies from IMDB. I counted the number of movies produced per year and need some help with the interpretation of two big jumps in the history (first is in the end of 20's and the second in the end of 80's). How can this be interpreted?
Yet the dataset is not complete, the documentation of the data says that
Movies were selected for inclusion if they had a known length and had been rated by at least one imdb user.
As Catija mentioned in the comments, in the late 1920s/early 1930s, feature-length films were likely becoming increasingly prevalent due to improvements in movie-making technology. Indeed - again, as referred to in the comments - the first feature-length talkies began to come out, starting with The Jazz Singer in 1927. (For a more detailed review of film firsts, you can take a look at my answer to an older question here - you'll notice that the 1920s get mentioned a lot.)
As for the spike you see in the 1980s and '90s, there may be some bias due to the criteria for inclusion on the graph (namely that at least one imdb user had to have rated it, and many users may be more familiar with '80s and '90s films than with earlier ones).
But if there's an explanation beyond that, it may be due to a couple of factors. First, as Wikipedia says, "The decade of the 1980s in film saw the return of studio-driven pictures, coming from the filmmaker-driven New Hollywood era of the 1970s." Second - again working off of Wikipedia information (sorry, I know it's a less-than-ideal source...) - films in the '80s were building off of an increase in popularity of certain genres that began in the '70s. Several genres experienced "booms" that decade, often with the help of popular movies from the '70s - sci-fi series like Star Wars and Mad Max began in the 1970s, and their sequels continued into the 1980s, often to wild success. This perhaps inspired much of the boom in sci-fi and blockbuster films during the rest of that decade, which in turn may have contributed to the apparent overall boom in movie production. (Though you'll also notice in Wikipedia's list of trends that many other genres saw increased attention and production, as well.)
You may also notice that the '80s seemed to see a spike in movie sequels - perhaps due to the popularity of the sequels (such as the Star Wars movies) that came out early in the decade. The realization that sequels could be huge money-makers would certainly explain a lot of the movie trends in the following decades, as well as the general spike in movie production in the '80s and '90s.
There are several huge problems using data from the IMDB for your analysis, especially with the caveat that you listed:
Movies were selected for inclusion if they had a known length and had been rated by at least one imdb user.
In the first two decades of the 20th century, most of the films produced were shorts, not features. Even many dramatic films were short films in this era. Starting in about 1915-1916, there was a huge spurt in feature film production by the major studios like Paramount. There were many, many studios back then, because Wall Street had not gotten into the movie business yet, and the industry had not consolidated into a few major studios. I don't see nearly enough features from this period in this graph. (Film production plummeted in 1918 due to shortages in film stock, manpower and raw materials like lumber. Also the flu epidemic greatly depressed audience attendance.)
Approximately 80% of the movies from the 1920s, and 90% of the movies from the 1910s are lost. No known copies exist. There are a large number of early sound films from 1929-1931 that are also missing, because they were not deemed good enough for re-issues in theaters and/or TV, and the studios did not preserve a copy. While the most famous lost films like CLEOPATRA (1917) and LONDON AFTER MIDNIGHT (1927) still have ratings on the IMDB, they are from users who have not seen the film. There are thousands of films from the silent era and the early sound era that don't have any ratings.
You don't mention whether you are counting American feature films or world-wide features. During World War I, European feature film production plummeted to almost nothing, which is why American studios became so dominant during and after the war.
The length information in the IMDB is extremely inaccurate for the silent era. There was not a soundtrack that required the film to be projected at a particular speed, so speeds would vary widely from cameraman to cameraman and theater to theater. The minutes listed in the IMDB for many silent features that have not been released on video are wildly inaccurate, or no time is listed at all.
The length of the average "feature film" has changed over time. During the 1910s, the average feature was 50-70 minutes, about the length of a TV show episode today. By the 1920s, most features were over an hour, but many prestige features were longer than two hours. In the 1930s and 1940s, "double feature" programs are very popular in theaters, and the second feature was almost always about an hour again.
Feature film production contracted in 1930-1932 due to the Great Depression and several studios going bankrupt. (The big studios didn't die, they were just taken over by the banks or other investors.) The cost of sound production put most of the independent producers and studios out of business. But I see no dip on your chart for these years.
A much more complete source of feature film quantity information for the American silent era would be the American Film Institute's online database at. They consider a feature film as any film over four reels (about 40 minutes), but not including serials.
