Monday, September 2, 2013

Quarterback Rating II: Let the Rookie Sit

Abstract
Many franchises use high draft picks on quarterbacks, rightly understanding their importance to a well functioning team. There is enormous pressure to start these players right away, but is that a good idea? Based on a player's peak quarterback rating, the answer appears to be no. Whether this is due to the pressure of the job breaking fragile young QBs or because the teams most likely to start a rookie passer are also most likely to have other problems is unclear, but either way indicates that teams should be wary about throwing young quarterbacks right into the fire.

Introduction
As mentioned in my last post, the quarterback is the focal point of every NFL offense; all offensive plays run through his hands. Unsurprisingly, teams place heavy emphasis on the selection and training of talent at the position. Promising (and even some not-so-promising) quarterbacks are hot commodities—since 1990 nearly 60% of first overall picks in the NFL draft have been QBs.

The pressure on these passers is intense, especially from teams which expect immediate production from their rookie signal-caller. Even teams who intend to let the new QB learn from the bench for his first year frequently find their plans changed by injuries or pressure from fans.

But is it good for young quarterbacks to get rushed into starting roles like this? There are certainly QBs that find success after starting as rookies - Peyton Manning comes to mind, and Russell Wilson and Robert Griffin III certainly seem to be in good shape. But for each success story there are plenty of high-profile failures.

Certainly for at least some of those quarterbacks there are other good reasons for why they didn't live up to expectations, but considering how important the position is the ratio of successes to failures seems frighteningly low. Of course, on this blog gut feeling isn't good enough; let's see what we can prove.

Data
Data once again comes from Armchair Analysis using the same queries as in the last post to compute seasonal QB ratings for every passer in the database.

It is important to note that while a truly heroic feat of data collection and organization, the Armchair Analysis database is not perfect. While inconsistencies appear to be minor (at worst) for most of the statistics, there seem to be somewhat larger issues with data on when players were drafted.

For instance, Carson Palmer is listed as being a rookie in 2004, but was actually drafted in 2003. The database also doesn't handle players taken in the supplemental draft very well. Overall, however, the data quality seems to be very good, and I'm confident that the results are not significantly biased by any typographical mistakes.

Results
In order to produce as unbiased a sample as possible, I restricted the investigation to quarterbacks who have at least four seasons, including their rookie season, in the database. Additionally, the quarterback must have thrown more than 150 passes in at least one of those seasons.

Determining a reliable measure for quarterback skill is a non-trivial task; ESPN's Total Quarterback Rating involves "several thousand lines of code" and the website I just linked implies that advanced computational techniques, such as machine vision, are involved. I (sadly) don't have the amount of time necessary to do something this complex, so I'll be sticking to the regular old-fashioned QB rating.

An additional roadblock comes from grading quarterbacks over their careers. As it turns out, a QB's passer rating in one season is a surprisingly poor predictor of their rating in the following season.

While it's clear that QB rating is an imperfect measure of a passer's skill, it still works reasonably well as an overall gauge of competence at the position. To try to avoid the year-to-year issues I'll only look at a quarterback's peak QB rating—their absolute best season.

Alright, that was a lot of explanation, now let's get to the good stuff. Figure 1 shows a histogram of the maximum QB ratings of the entire sample (in gray). The majority of signal-callers in the sample have peak QB ratings between ~75 and ~90, with an average peak QB rating of 83. But there's a fair amount of variance in the sample, from 55 (Mike McMahon, who managed to start seven games for the ill-fated 2005 Eagles) to 118 (2011 Aaron Rodgers, who actually got a rating of 123 in the regular season but this data includes his poor performance in the Packers' playoff loss).

Figure 1: Peak QB ratings of the sample.
I've broken this sample down in two ways. First, I've selected all quarterbacks who threw 150+ passes in their rookie year (purple histogram). I next select QBs who have met the passing criterion for at least 4 seasons (gold histogram). While not perfect, passers who stick around in the NFL for several seasons are going the be the best and most reliable quarterbacks, so length-of-tenure is a good proxy for the most skilled playeres.

The results are notable—the 4+ year starters have a uniformly higher peak QB rating than the group who saw significant action as rookies. In fact, the veteran QBs are responsible for all seasons with a QB rating above ~90, while no passer is given 4+ years in the league as a starter without at least one rating above 70.

Not only are these two distributions notably different, they're significantly dissimilar. The standard test to see if two samples of data come from the same underlying distribution is the Kolmogorov-Smirnov test (usually abbreviated as the KS test). This test says that the two sub-samples are distinct with only an 8% chance of error. However, we know that the rookie starter and long-tenured QB distributions actually do come from the same parent distribution. In addition, there is clearly overlap between the two sub-distributions.

The net result of these facts is that there is an even smaller chance of error than the KS test would indicate. A full Monte Carlo simulation indicates that there is actually a 99.3% certainty that these two distributions are distinct.

Discussion and Conclusions
This result, in its barest form, means that the conditions that result in a quarterback starting as a rookie are different from those that lead to success in the NFL. Note that it does not necessarily mean that a rookie quarterback will not have a long, productive career; there is some overlap between the two sub-distributions. Nor does it mean that the solution to the problem is to make sure all rookie QBs stay far away from the field; the teams that are in a position to redshirt their first-year passers are also the ones most likely to already be in better position to protect their investments when they do make it to the field—for instance the Packers with Aaron Rodgers or the Patriots with Ryan Mallett.

But despite these caveats, these findings are still quite interesting, and indicate that quarterbacks who see significant rookie action tend to have lower ceilings than signal-callers who are rested at the start of their careers. This result indicates that teams who are looking to use a high draft pick on a (hopefully) franchise quarterback should resist the urge to play him right away, and maybe consider upgrading their other positions of need first before drafting their QB of the future. So the next time your favorite team passes on a flashy gun-slinger in order to draft a boring left tackle, don't judge them too harshly.

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