There are a few positions in the NFL for which straight-line speed seems to matter. Chief among these is running back; does "Speed Score" ring a bell? Well, I'm putting together a running back projection model for Football Outsiders Almanac 2013 (only a month or so away!), and part of the research naturally involves how to handle 40-yard dash times.
More precisely, it involves figuring out how to handle 40-yard dash times at pro days. Conventional wisdom says that these times aren't trustworthy because they don't happen in an objective environment like the NFL Combine: Practically everyone in attendance (pro scouts, athletic department officials, college coaches, the players, etc.) has a clear motivation to engage in a shenanigan or 20.
Is conventional wisdom correct? To find out, I looked at every running back prospect from 1999 to 2013 who met the following criteria:
- They played their final year in an automatic-qualifying FBS conference (or at Notre Dame).
- They were not a fullback or a potential position convert.
- They were either drafted or went undrafted despite being among NFL Draft Scout's top 50 at the position.
- They ran the 40-yard dash at the combine.
- They ran the 40-yard dash at their pro day.
That produced a sample of 99 running backs whose twin 40-times I could use to address the question, "Do backs have faster times at their pro day than they do at the combine?"
It turns out that the average 40-time at the combine for this group was 4.58 seconds; for pro days it was 4.55 seconds. That's not a huge difference, but a one-tailed, paired samples t-test suggests that the probability of it happening by random chance is less than 0.001 percent.
Now, you may have noticed I was careful to use the term "backs have faster times" instead of "backs run faster." Actually running faster is certainly a possible explanation, but so too are a whole host of other things (e.g., shenanigans). Obviously, I can't address this "why" question directly, but I did find something in the data that hints at potential explanation indirectly.
Namely, for 75 of the 99 running backs, we also have 10-yard and 20-yard splits for both venues; with some basic math, we also have the so-called "flying 20." If backs are simply running faster, we would expect that 0.03 average improvement to be spread out across the various splits in a systematic way. For all running backs (not included above) who ran at the combine since 1999, the 10-yard split accounted for 35 percent of their 40-times, the 20-yard split accounted for 58 percent (which means the second 10 accounted for 23 percent), and the flying 20 accounted for 42 percent. When divvying up 0.03 seconds, we're firmly in "distinction without a difference" territory, so I'm not silly enough to expect an average pro day improvement of, say, 0.02 seconds for the first 20 yards and 0.01 seconds for the flying 20. Nevertheless, we should see something approximating these splits.
The first half of the run is pretty much in line with our rough expectations. The 75 running backs had an average 10-yard split of 1.57 seconds at the combine and 1.57 seconds at their pro days. Similarly, the average 20-yard split was 2.61 seconds at the combine and 2.62 seconds at their pro days. Obviously, neither of these are statistically significant differences. And although we probably should have seen some improvement in those two splits, I'm not inclined to quibble over a hundredth-of-a-second or two.
The flying 20 is a different story, one worth gratuitous amounts of quibbling: The average drops from 1.96 seconds at the combine to 1.91 seconds at pro days. Per a t-test, the probability of that result being due to random chance is less than 0.00001 percent, which is even lower than our finding for the 40-yard dash as a whole.
Maybe, despite what a classical statistics test says, this is still just rounding error randomness; or maybe Chase Stuart is out there somewhere yelling, "splits happen!" My hunch, though, is that the conventional wisdom is right -- kind of. Yes, 40-times do appear to be faster at pro days than at the combine, and it's probably due in some part to the human element. However, our disbelief should only be focused on the flying 20, where the human element has tended to develop a spontaneous itch in its stopwatch trigger finger. Do I have definitive proof of this? Of course not. But I also don't think the average running back prospect can fly faster at his pro day when he just ran 20 yards at combine pace; it's only slightly more believable than seeing pigs fly beside him.
What do you think?
In case you're wondering about other timed drills, the average back from an automatic-qualifying conference ran his pro-day short shuttle 0.03 seconds faster than he did at the combine, and ran his three-cone a whopping 0.09 seconds faster. My guess (with tongue firmly in cheek): It's got something to do with the big cone.