Tiki Barber recently called Adrian Peterson "a liability for his team" due to his propensity to fumble. Is that assessment fair? What if Peterson's fumbles really did hurt his team more than his overall performance helps? Peterson himself admits he has a fumble problem, and he's promised to work on it in the off-season. How many fumbles is too many when it comes to a break-away home-run hitter like Peterson?
I'll look at Peterson's fumbles in three contexts. First, I'll look at his rumble rate, which accounts for how often he's asked to carry the ball. Second, I'll look at how costly his fumbles have been in terms of Win Probability Added (WPA). And third, I'll look at his fumbles in terms of Expected Points Added (EPA), which is less sensitive to game situation as WPA.
Aside from his recent fumble problems in the NFC Championship Game, Peterson has committed 4, 9, and 7 regular season fumbles over his three-year career. That's certainly more fumbles than you'd like to see, but keep in mind how often he's asked to carry the ball.
Any fumble is equally likely as any other to result in a turnover or recovery, so I'll start by looking at fumbles rather than fumbles lost. Further, fumble rate is going to tell us more than total fumbles about a player's proclivity to lose the ball. For RBs, especially guys who get a lot of receptions, it makes sense to consider their total touches, which includes carries and receptions.
Fumble rate is extremely random from season to season. For RBs with at least 60 touches in a regular season, fumble rate correlates with the following season at only r=0.04. Fumbles are rare, low frequency events. One or two more or fewer instances in a year can result in an apparently large increase or decrease in fumble rates. Correlations are going to be very low in general, but some guys do show a knack for fumbling.
In 2009, Peterson's seven fumbles came over the course of 357 touches, for a fumble rate of 2.0%. That's one fumble for every 50 touches, and a 50% nominal recovery rate would mean one fumble lost for every 100 touches. His rate in 2008 was 2.3%, and in 2007 it was 1.5%. The league average for RBs with over 60 touches in a season over the last three years is 1.3%, so Peterson has been consistently worse than average.
The graph below plots the distribution of RB fumble rates over the past three seasons. In other words, it shows how many RBs fell within each 'bin' of fumble rate. Most backs have a rate between 1.0 and 1.5%, but plenty have more. Each of Peterson's three seasons were within one standard deviation from the mean. Although worse than average, he's far from the league's worst.
Part of the frustration Vikings fans (and fantasy owners) felt with Peterson in 2009 may have to do with the unusually high share of his fumbles that were lost. Six of his seven fumbles in the regular season resulted in turnovers, one of them returned for a touchdown to tie the score. In total, 13 of his 20 career regular season fumbles were lost. But RB fumbles are typically recovered 50% of the time, and recoveries are largely out of the RB's control.
Peterson also seems to have a habit of losing fumbles in critical situations. In his three seasons in the NFL, including playoff games, Peterson accumulated +6.94 Win Probability Added (WPA). In other words, the plays in which he has participated were worth nearly 7 wins above average for the Vikings. Unfortunately, his fumbles, both lost and recovered, have cost -2.42 WPA over the same period, for a net of +4.52 WPA. That's a loss of 35% of his gross productivity due to fumbling.
To put that 35% in perspective, I sampled a few comparable RBs. Chris Johnson has lost 5% of his positive productivity to fumbles. Ray Rice has lost 16%. Michael Tuner and Marion Barber each lost 8%. Clinton Portis came the closest of the group, losing 23% of his productivity to fumbles.
EPA tells a similar story, (but for reasons I'll mention below, we have to take EPA for RBs with a grain of salt). Peterson's fumble-free production totaled +105.9 EPA. For a RB, that's a lot of points above-average over 29 games played. But he has forfeited -76.2 EPA, or about 71% of his gross positive production, to fumbles.
Frankly, 71% sounds astonishing, but it's worth taking a minute to discuss what EPA actually is. EPA is marginal point production above "expected," which is the league-average baseline. It's comparable to a corporation's profit, rather than its overall revenue. Marginal quantities like profits, when compared to the underlying total, are always more sensitive to calculations and comparisons. A doubling of a company's profit may only be due to a 1% increase in its baseline revenue. Both EPA and WPA are sensitive to the same kind of comparisons.
Also, RBs are going to be unfairly penalized by EPA. Due to an apparent fundamental imbalance, running is simply not as productive as passing in terms of Expected Points. Runs on 1st and 2nd down outside the red zone, which comprise the vast majority of rush attempts, tend to be negative events in the eyes of EPA. The average RB is therefore going to have a negative total EPA, and any RB with positive EPA is solidly above average.
But EPA can be even more problematic. According to traditional stats, Peterson's best year was 2008 when he totaled 1,760 rushing yards at a 4.8 Yard Per Carry clip. It's hard to deny that's good, but his EPA that season was actually slightly negative. Fumbles were the main culprit, but there was another drag on his EPA. In games where the Vikings had a lead, he was handed the ball for lots and lots of 1, 2 and 3-yard gains, which are negative plays. WPA may see these carries as neutral or positive because they burn clock, but EPA only sees reductions in the expected net point advantage.
To be fair, we need to compare a RB's EPA to that of other RBs. For example, Ray Rice lost -24.3 of his gross +52.5 career EPA (46%) to fumbles, and Chris Johnson lost -11.1 of his gross +50.0 EPA (22%) to fumbles.
Peterson is not alone as a fumbler, as other great runners have had similar problems. Walter Payton posted a 2.0% fumble rate over his career. Franco Harris fumbled at a whopping 2.8% rate. Tiki Barber was known for his fumble problems, but also for fixing them by the end of his career. Barber's overall career rate was 1.9%, but through the bulk of his career, from 1999 through 2003, his rate was 3.1% (Caution: multiple endpoints argument!). In Barber's final three years, he reduced his rate to 0.9%. Some other notable backs weren't as clumsy: Emmit Smith--1.2%, Edgerrin James--1.3%, Jamal Lewis--1.4%. Brian Westbrook is the counter-example, with an enviable 0.7% career fumble rate. (Historical data from PFR.)
So, yes, Adrian Peterson does have a fumble problem, but much of it has to do with the situational context in which his fumbles were lost. If Peterson could control when they were lost, I'm sure he'd choose to fumble only when it wouldn't matter. But he obviously can't do that. All he can do is improve his fundamental techniques to reduce his overall likelihood of fumbling.
The real question is: given his costly fumbles, is he a liability? According to WPA, he's not. Theoretically, the average RB would net slightly below 0.0 WPA. Peterson's career net +4.52 WPA means he's been a net positive in helping his team win. Fewer turnovers would be nice, and although it must be frustrating to teammates and fans alike, even if Peterson doesn't improve his fumble rate he's still worth it. Still, imagine how valuable he'd be if he could reduce the fumbles.