Last week, I looked at Kevin Kolb as the patron saint of quarterback changes, and what happens to passing statistics when a team has multiple starting quarterbacks. Today, I’m going to look at it slightly differently. We can also see how things change when things stay (roughly) the same. When one quarterback starts every game, how consistent are they in different categories? Over the last five years, there have been 75 occasions when a quarterback started all 16 games in a season.
I split those seasons in half, and compared the first eight games to the final eight games, looking at the five rate categories (completion percentage, yards per attempt, touchdown rate, interception rate, and sack rate). If you recall from my earlier post, sack rate and interception rate showed the most inconsistency when a team changed quarterbacks.
Russell Wilson is a lesson that not all big changes have to come with big changes at the quarterback position, and there is plenty of variation in an individual athlete’s performance thanks to gaining experience, the impact of schedule, changes in the offense, and teammates. The 2012 splits for Wilson were the largest combined across all five categories. The most consistent was completion percentage, where Wilson shot up from 61.4% to 67.2% between the first half and second half of the season. Only one quarterback had a larger split in Yards per Attempt in the sample; Wilson went from 6.98 to a MVP-like 9.0 yards per attempt over the last 8 games. Wilson’s touchdown rate almost doubled, from 4.8% (4.3% if you live in Green Bay) to an astronomical 8.7%, his interception rate was a third of what it was the first half of the year, and the sack rate climbed from 6.3% to 9.0%. If I had shown you a blind comparison of Quarterback A and Quarterback B, there is no way you would have guessed they were the same quarterback from different halves of the same season.
He is the extreme example, though. What does it look like when we compare the same quarterback across a single season? Here is the summary of how frequently each quarterback’s performance from first half to second half was within one standard deviation, between one and two standard deviation’s difference, or showed a greater than two standard deviation swing in the rate categories.
Yards per Attempt checks in as the most consistent category, followed by Completion Percentage. These are the two categories that are the least susceptible to a couple of plays swinging the results. While the range in every season was slightly different, it took about a 0.75 change in Yards per Attempt or a 4% change in completion percentage to equal a one standard deviation change in the category. One screen pass turned into a long touchdown or a couple of drops aren’t going to have the impact that a couple of tipped interceptions could.
Among the three “rare events” categories, sack rate checks in as the most consistent and interceptions the least. While it is not a straight passer to passer comparison, since not everyone in the previous group played exactly 8 games, and the splits weren’t uniform, it is still interesting to look at how the changes in performance compare between the two groups.
Here is the percentage of performers in each category who stayed within one standard deviation, depending on whether the team changed quarterbacks, or the same quarterback started every game.
While interceptions are largely random, having the same quarterback showed far more consistency than what happens when a team changes quarterbacks. Sack Rate also shows more stickiness, with a difference of over 10%. Interestingly, the touchdown rate was more inconsistent when the same quarterback threw the passes in each half season than when pairs of quarterbacks on the same team were compared. Completion percentage also showed basically the same percentage of passers staying relatively similar.
Here is a similar chart, but showing the percentage of time that a huge swing (> 2 S.D.) occurred.
Sack rate was three times more likely to show a massive swing when a team changed quarterbacks, again evidence that the quarterback plays a large role in determining that number. The interception rate was almost twice as likely to change drastically when a team went a different route. Part of this may be self-fulfilling. Not all of those times a team changed quarterbacks were for performance (injuries certainly play a role), but one of the most noticeable reasons a quarterback is benched for poor play is excessive turnovers. The population of quarterbacks who start every game, meanwhile, is filled by mostly pretty good quarterbacks with at least decent interception rates.
While completion percentage had a tendency to stay consistent whether the quarterback changed or not, the truly big swings occurred only when a team changed quarterbacks. The biggest change occurred in 2009, when Philip Rivers shot from 60.6% to 71.2% in thye second half of the year. 69% of the quarterbacks completed passes at a rate within 5% of their first half splits, compared to the second portion of the season. Only Rivers and Peyton Manning in 2008 had a swing of over 10% in their completion percentage within different halves of the same year.
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[photo via USA Today Sports Images]
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