First let me thank Greg Trippiedi at Hog Heaven for inspiring this post, as his analysis of the offensive line spawned a discussion of The Mustache vs. Mark Brunell on third down. The entire read is worthwhile for what it adds to the discussion on our O-Line last year, but I focused on this portion:
It's one thing to claim that, it's another thing to prove it statistically. I believe what I found supports my theory that Mark Brunell left too many chains on the field by completing the wrong kind of passes. Statistically on third down, per his passing splits, Mark Brunell had 84 attempts on 3rd down. 30 of those passes resulted in 1st downs, which is a conversion rate of 35.7%. The Redskins third down conversion % in 2006 overall was 37.4%, for context.
Jason Campbell, to contrast, had 70 third down attempts and converted 26 of them, for a 37.1% completion rate. Demonstrably he was more productive on third down.
This doesn't tell us everything. For instance we don't know what kinds of 3rd downs JC was facing relative to Mark Brunell; they might have been shorter distance, for instance. And the difference in conversion % doesn't seem that drastic.
But there's more. The perception was that Mark Brunell wasn't merely failing to convert 3rd downs (which he was, at a rate less than both JC and the team generally) but rather that he was completing passes that weren't going for 1st downs. Consider that Jason Campbell was more productive on 3rd down despite having a lower completion % (48.6% vs. MB's 56% on that down) and QB Rating (70.3 vs. MB's 89.6). That should partially tell us how instructive QB Rating is in evaluating quarterbacks, but I digress...
Mark Brunell completed 47 passes on 3rd down, but just 30 of them converted. That's a 63.8% conversion clip, which sounds good but is actually quite weak. If you complete your intended pass on third down, it should nearly always yield a 1st down, as presumably you're selecting completable passes that somehow find the ball moved past the chain. The Mustache Master completed 34 passes on 3rd down that moved the chains 26 times, or a 76.4% conversion clip.
Throw in the fact that JC converted over 50% of his 3rd down runs for 1st downs, and MB converted less (with a lost fumble among his 5 rushing attempts on 3rd down), and that Campbell was sacked just 4.2% of the time on third down vs. Brunell's 7.1% (despite his frustrating tendency to simply dump the ball to whomever was nearest), and I'm a happier camper with Jason under center on 3rd down than I am with Brunell, higher completion % and QB Rating be damned.