Before looking at the numbers though, we need some rules to clarify what types of movies we are considering:

• We only consider theatrically-released films — no straight-to-video movies or TV movies.
• Short films that were released theatrically (such as Pixar’s Presto) are included.
• We only consider movies that have received 1000 or more votes. This restriction is to prevent movies with only a handful of votes from skewing the results too much.
• The theatrical release date of the movie must have been at least as recent at 1950.

IMDb contains 10034 movies that satisfy the above criteria. The average score (on a scale of 1 to 10) of those movies is 6.38 and the median score is 6.6. The average score per release year is given by the following graph:

As you can see, older movies (1950 – 1975) have abnormally high scores, as do very recent movies (2000 – 2009). These differences are indeed statistically significant. For example, the p-value associated with the test that the mean score in 1950 is the same as the mean score in 1989 is less than 10^-19. The p-value associated with the test that the mean score in 2008 is the same as the mean score in 1989 is about 0.0021. Other nearby years give similar p-values.

So this tells us that, in general, particularly old movies receive the highest scores, followed by newly-released movies, followed by “semi-old” movies from the 1980′s and 1990′s. So why the differences? Were movies from the 1980′s really just that bad? Possibly, but the more likely explanation is that movies from the 1950′s  through 1970′s have artificially higher scores because people don’t generally go back and watch the crummy movies of the last generation, so they get forgotten and do not have 1000 votes on IMDb. Will people be watching Disaster Movie in forty years? I sure hope not.

On the other hand, particularly recent movies tend to draw a fair amount of hype and fanboyism. Remember when The Dark Knight had a score of 9.8 and was at #1 on the IMDb top 250? Now, one year later, it has a score of 8.9 and is located at #9 on the top 250. It will likely dwindle a little further down over the coming years as well.

The Best and Worst of Each Year

While we’re looking at ratings of movies over the years, I suppose I might as well provide a list of the best and worst movie of each year (based on the votes of IMDb users), since such a list is not available on the IMDb website itself to my knowledge. Keep in mind that, as before, only movies with 1000 or more votes are considered. Enjoy!

Downloads:

• IMDb rating data [.zip of an Excel spreadsheet -- 341KB]
• IMDb rating data [tab-delimited plaintext -- 0.97MB]

Up's balloon house

Let me say right off the bat that I am in no way insulting or belittling the new Pixar film Up, which was released in theatres this weekend; it was one of Pixar’s better outings, and that’s saying something. However, after reading a ridiculous article on PopularMechanics.com that actually defends the physical plausibility of Up, I couldn’t help but write a thing or two about the (im)plausibility of the movie myself.

The absurdness of the article appears as early as its fourth sentence:

Given the fact that one cubic foot of Helium can lift 60 pounds, with even small, 1 x 1-foot balloons, Carl’s rig has the capacity to lift about 618 tons—enough to lift about 150 Hummers.

- PopularMechanics.com

Let’s think for a second about the numbers in that sentence, shall we? One cubic foot of helium can lift 60 pounds. Really, PopularMechanics? When was the last time that you were at a fair and saw a full-grown man get carried away by four helium balloons? How about instead of putting window washers and construction workers on scaffolding, we just tie a handful of balloons to their waist? Seems like a more economical solution to me.

Indeed, since PopularMechanics.com can’t seem to be bothered, let’s do some actual math to figure out how plausible the situation in Up really is. Helium’s density is about 5.06 grams per cubic foot and the density of air is roughly 36.11 grams per cubic foot. Thus, one cubic foot of helium can lift roughly 31.05 grams (this number will vary slightly depending on what figure you use for air density and whether or not you include the weight of the balloon itself, but I think that we can all agree that it is slightly less than 60 pounds).

Assuming that the average balloon indeed holds a cubic foot of helium (this seems like a reasonable assumption to me), we then find that it would take some 2630 balloons just to lift a 180-pound old man. To pick up his house (assuming a weight of 50 tons, which seems reasonable to me) as well would then require some 1.46 million balloons—a far cry from the reported 20622 balloons that were actually used. But hey, 20622 balloons would be enough to lift about 1400 pounds, or just under 3/4 of a ton. That’s pretty close to what you said, right PopularMechanics?

Update [June 1, 2009]: PopularMechanics has now corrected their article by replacing the offending text with “Given the fact that Helium can lift over six times its weight, Carl’s idea isn’t entirely fiction.”