Cinematic Grade Inflation – Part II

A brief follow-up on my previous post about Cinematic Grade Inflation. In that post, I noted that the average grade of the top 100 rated films (with at least 20 reviews) on Rotten Tomatoes had risen substantially from 2000 to 2009.

Following up on my post, Scott Macaulay over at the Filmmaker Magazine blog asked an interesting question:

I’d be curious to see the sample set of critics analyzed over the decade. I bet it’s a lot larger now, and I wonder if the new breed of critic is more disposed towards positive reviews than the critics we entered the decade with.

In an effort not to let that question be rhetorical (and out of personal curiosity) I decided to follow up and take a look.  What I found was not quite what I expected and, frankly, asked more questions than it answers:

So the average number of reviewers was higher in 2009 than in 2000, but if you look at 2000-2008 it actually appears to drop. Also, what was the reason for the extremely high average in 2004? And why so low in 2008?

Any ideas?