Monday, August 19, 2013

Quarterback Rating I: Year-to-Year Progression

Abstract
Using quarterback ratings I've charted out a QB's average improvement from his first season as the starter. On average a QB sees only a minor ~10-point rating boost in his second year, with his rating remaining flat (or lower) for the rest of his career. Additionally, very few (~20%) players will ever have a season with a QB rating more than 20 points higher than their first year. These results indicate that a quarterback's first season is a reliable indicator of their future success, and that passers who struggle in the early stages of their career are unlikely to show significant long-term improvement.

Introduction
As the guy responsible for handling the ball on every single offensive play, the quarterback is unambiguously the most important player on a team. So when a team drafts a new quarterback the pressure is extremely high - both on the player to perform to expectations and on the management to ensure they're getting a good return on their (significant!) investment.

In recent years QBs have been asked to step in and start as rookies with increasing frequency. Last year saw a record 5 rookie signal-callers taking the majority of their team's snaps. While this year's draft appears to have a definite lack of QBs ready to start immediately, it's a virtual certainty that a few desperate teams will roll the dice on their shiny new gunslingers.

With the importance of the quarterback position and proliferation of young, untested starters, it's critical for teams to accurately evaluate QBs, not only as college prospects but even while they're playing in the NFL. While there is clearly worth in exploring how quarterback talent is evaulated for the NFL draft, the sheer number of college teams, and the limited opportunities given for the best players to play against each other, make it very difficult to perform such an analysis without more advanced tools.

Fortunately, charting the progression of quarterbacks once they enter the NFL is also interesting, and somewhat easier due to the small number of teams and the high level of competition. A good manager is always watching how their players progress, and it's highly relevant to know whether a struggling QB is merely inexperienced or a hopeless cause. There are several ways to dig into this topic: for now I'll focus on computing the average year-to-year progression of NFL quarterbacks as a general barometer for how a QB should be expected to develop.

Data
As usual the data come from the Armchair Analysis database. I first queried the database for the identifiying information for all QBs, then fed that into a query which returned all game stats for each QB. From there season totals were computed.

Finally, the seasonal QB rating for each quarterback was determined. Because the QB rating can be highly biased if a passer only has a small number of attempts in a given year, I only took ratings from seasons in which the QB threw at least 150 passes.

Results
A (relatively) simple way to track a signal-caller's improvement over time is to compare their QB rating from a given season to earlier seasons. An aggregate plot comparing a passer's QB rating from later seasons to their first 'full' season (full being a season where the QB attempted at least 150 passes) is shown in Figure 1. The data are shown as black points, while the averages (and standard errors) are shown in red.
Figure 1: QB rating improvement from first season as a function of years in the league. Red points show average improvements.

While there is significant scatter, it is clear that on average a QB only shows improvement between their first and second full seasons. After that, performance stabilizes until the 7th season or so, where it begins to decrease (although the data appears to show that the few QBs who make it to their 10th season are able to maintain their improved performance).

This performance boost, at only 5-10 rating points, is moderate at best, and indicates that a quarterback's first full season is a strong indicator of their future success. Of course, this is only an average and as such somewhat of an abstraction - clearly not all QBs will follow exactly this trend.

To gain more insight into the maximum potential improvement over a quarterback's career I've plotted a histogram of peak QB rating improvement (or minimum reduction, the sad reality for some passers) in Figure 2. It's clear from this figure that the majority of signal-callers never progress beyond a 20-point improvement in QB rating, even during their best seasons, with only 20% of all passers in the sample beating this threshold1.
Figure 2: Histogram of peak QB performance compared to a player's first starting season.

Discussion and Conclusions
Even at their very best, this analysis shows that most quarterbacks shouldn't be expected to show dramatic improvement at any point during their careers, and only moderate improvement from their initial starting season. This analysis indicates that even a rookie QB's ceiling can be estimated with reasonable certainty, and has clear ramifications for evaluating quarterbacks. For instance, this is bad news for Andrew Luck (first year QB rating of 76.5), Ryan Tannehill (76.1), Jake Locker (74.0), and Brandon Weeden (72.6), who are all unlikely to ever see a triple-digit rating but are tabbed as the starters heading into 2013.

These results also lend credence to the arguments of impatient fans, who expect to see immediate results from new QBs and have no patience for any 'adjustment period', 'learning curve', or any other excuse offered by a team for a young passer's poor play. I had always assumed these fans were merely short-sighted, unwilling to wait and see how a player would develop. But now it's much more difficult to dismiss their concerns so easily.

1: The two players in the sample with a 40+ point QB rating improvement? Alex Smith and Eli Manning. 

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