Quarterbacks: Starters verses Backups

By Denis O'Regan.

This is a tentative attempt to find a ball park figure for how much a team's chances are compromised when the backup quarterback plays.

Firstly,I collected and combined data for every regular season play since 2007 made by a quarterback who,at the start of the season was considered to be the team's number one quarterback.

I then repeated the exercise for every other quarterback who threw a pass over the same time scale.As I'm initially just looking for a general figure I didn't take any account of how the backup came to be playing.Backups can be under centre for a variety of reasons.They can be mopping up at the end of a large victory or playing out time after a heavy defeat.The best comparison between backups and starters is obviously when the latter is injured and the former starts,but for the moment I've looked at all plays.

I did eliminate passes thrown by non quarterbacks,such as punters or running backs,but I'll present those numbers at the end because they do make interesting reading.

I looked at various passing stats,but I'll be using yards per attempt as the main tool to compare an average backup with an average starter.

Here's the results.

Interceptions.As you'd expect the average starting QB looks after the ball better than his understudy.Backups are intercepted on 3.6% of their attempts since 2007 compared to only 2.8% for starters.

Touchdowns.Starters throw a touchdown on 4.3% of their attempts compared to just 3.3% for backups.

Sacks.Once again starters are better,going down on 5.6% of their total dropbacks compared to 7.3% for backups.However, backups do slightly better by only losing 6.3 yards per sack compared to 6.5 yards for starters.So a small,if insignificant victory,although as we're only looking at two seasons worth of results,this could simply be a sample size issue.

Quarterback rating.I know it's not perfect,but I'm including it anyway.If you take every play made by a starting QB since 2007 and compile a combined QB rating you get 86.0.For backup's it's only 72.4.

Completion percentage.Not surprisingly,backups do poorly compared to starters,not only do they lack the starter's talent,they also get much less time to practise with their receivers.The backup averages a completion rate of 57.9% compared to 62.1%.Breaching 60% completion rates seems to be a factor that defines quality in a quarterback.Eli Manning only hit 56% of his passes in the Giants Superbowl winning season,but on the road,where the trophy was won he connected on 61% of his throws.Prior to Manning you have to go back to the Raven's Trent Dilfer (59%) in 2000 to find a SB winning QB whose regular season completion rate was sub 60%.

Yards/Attempt.This is the stat I'm going to use to estimate a starters worth.Backups pass for 6.26 yards per attempt,very nearly a yard per attempt less than starters,who averaged 7.11 ypa over the two season.These figures don't take into account sacks as failed pass attempts,nor does it subtract sack yards or interception yards,but if you do correct for these occurrences,then the discrepancy in the two figures remains fairly constant.

If you patch all these stats together you find that the 2008 quarterback who comes closest to matching the average stats for a starting quarterback is Denver's Jay Cutler.The pin up guy for the backups based on 2008 is Marc Bulger,which probably says as much about the Rams as it does about Bulger.If you want a player who comes closest to replicating the average backups stats who actually is a backup,try a combination of Brian Griese from 2007 in Chicago and 2008 in Tampa.

Armed with the comparison between yards/attempt for each type of quarterback I next constructed a predictive model based around yards per pass and yards per rush.
I calculated the yards per pass and yards per rush each team had achieved on offense and defense prior to each game from week 4 onwards for the last 7 seasons.I corrected these figures for strength of opponents faced.I then matched each teams offensive numbers with their opponents defensive numbers,and vice versa for every game played and regressed those numbers against the actual result.

For example if team A was averaging 6.5 yards/pass against defenses who were allowing 6.8 yards/per pass,then they were considered below average to the tune of 0.3 yards/pass.If they were matched up against a defense that had allowed 6 yards/pass against offenses that were averaging 6.4 yards/pass,then I considered that the defense they were facing that day was 0.4 yards/pass above average.Combining these two figures gives an overall projected passing capability of team A in this particular game of 0.7 yards/pass below the league average.

I also did this for the projected rushing offense of team A and repeated the process for team A's opponents on that day.

Both teams,therefore had a pregame projected rating for projected passing ability (containing information about their game day opponents pass defense) and rushing ability (containing informtion about their gameday opponents run defense).These four pregame inputs turn out to be statistically significant in predicting the actual game outcome and tests on out of sample games perform to a level similar to the Vegas line.This limited model also suggests that it is much more important to be able to pass the ball than it is to run the ball,which backs the intuitive knowledge that losing your starting passer is a really big deal.

Having produced a serviceable prediction model based on passing yardage all we need to do now to predicted the impact of a backup QB starting is to reduce the expected passing ability by a similar drop to the one seen in the two sets of aggregate stats for starters and backups.

If we do this for every matchup over a series of seasons we find that on average the presence of a backup for the whole game decreases the win probability of that team by about 8 percentage points.In other words a team with a win probability of 58% would turn into a coin toss if they played their backup in that game and their yards/pass numbers were reduced by just under a yard/pass.

So to sum up,in terms of points on the scoreboard a backup QB on average seems to cost a team about a field goal.Individual teams will of course see differences within these averages.Swapping between Manning and Sorgi you would expect would cost the Colts more than the average,whereas the choice between Orton and Grossman may result in little difference.

For completeness,here's the 2007-2008 stats for non QBs passing the ball.They completed 55% of their passes,they were sacked on 14% of their drop backs.They threw 10 yards/attempt and 23% of their passes went for TD's!.They had a combined QB rating of 116.