The Importance of Field Position

I'm currently reading a copy of KC Joyner's Scientific Football 2007. It's an excellent annual prospectus full of useful stats and analysis, which I highly recommend. KC regularly writes for ESPN Insider. I just read a part of Scientific Football entitled Straight from the Dept of Meaningless Statistics (p.210). KC suggests that average drive starting position is "not meaningful at all." He cites an example comparing the Steelers and Browns in 2006:

"PIT's starting field position was their 26-yd line, which was the worst in the NFL. Cleveland's average starting field position was at their 30-yd line, which was the 2nd best...If the average team has 12-13 drives per game, that would mean each team would have [approximately] 100 drives after 8 games. This means CLE would have a 400-yd advantage in this category.

"Four hundred yards sounds like a lot but let's also put it into prespective. If the Browns had 70 potential field position yards on each of their drives (i.e. they had 70 yds to go for a TD), that would mean they have 7,000 yds to go versus PIT's 7,400. Four hundred yards sounds like a lot until you consider that is in the context of needing to gain 7,000 yards."

At first I began to think about the question this way: 400 out of 7000 yds is about 5%. That's not that much, but as a Ravens fan, I'd gladly accept an extra 5% performance boost to my team's offense. But then I realized that field position is not linear, and percentage would not the best way to conceptualize it.

Think of an offensive drive not in terms of a series of passes and runs, but in terms of a chain of first downs, regardless of how they are achieved. To arrive in scoring position a team needs not just yards, and not just 1st downs, but consecutive first downs. The success rate for achieving a 1st down on each series has been 65% over the past 5 years. So a team's probability of sustaining a scoring drive is 0.65^x, where x is the number of 1st downs needed (which would include the final scoring series as well).

On average, an NFL offense needs 3.7 first downs (including the score itself) to score a touchdown. Therefore, the estimated TD rate would be 0.65^3.7 = 0.20 TDs per drive. (Note: The actual share of drives that resulted in touchdowns over the past five years is very close--19%.)

One way to think of those 4 extra yards is that they would typically require 0.4 more first downs to score. The resulting effect on the probability of scoring is 0.65^4.1 = 0.17. The difference is 0.20-0.17 = 0.03.

A difference of only 3% in the chance of scoring a TD on a typical offensive drive may seem very small, but it has a large impact on points. Given a league average of 12.4 drives per game (according to KC Joyner), the effect on two teams with a 4-yd difference in starting field position would be:

0.17 * 12.4 = 2.1 TDs per game (14.7 points)
0.20 * 12.4 = 2.5 TDs per game (17.4 points)

The result is a 0.4 TD per game advantage to a team with a 4-yd field position edge, the equivalent of 2.8 points per game. But it wouldn't work out exactly that way, because there is obviously no such thing as 0.4 touchdowns. So sometimes a team would end up with an additional TD, sometimes not, but perhaps sometimes 2 additional TDs. In my view, this effect is very meaningful.

Here is perhaps a simpler way to conceptualize it. Instead of saying the team with lesser starting field position needs 0.4 more 1st downs per drive to score, we could say that they need a full additional 1st down in 40% of its drives.

The resulting probabilities of successful TD drives are somewhat simpler to understand. This time I'll say the average # of drives per game is 10, which I believe is closer to the actual number than KC's 12.4 number.

0.65^3.7 = 0.20 probability of TD drive
0.20 * 10 drives/game = 2.1 TD drives/game

0.65^3.7 = 0.20 probability of TD drive
0.65^4.7 = 0.13 probability of TD drive
0.20 * 6 drives/game + 0.13 * 4 drives/game = 1.7 TD drives/game

Again, the difference is 0.4 TDs/game.

But we still need to consider field goals. Drives that stall just shy of the end zone are typically converted into field goals. So the estimated difference in expected points due to touchdowns would be mitigated by the expected consolation of 3 points for the team with the worse starting field position. That is, until we consider that the team with better starting field position would also get into field goal range easier themselves. The effect of field goals is essentially a wash.

This is a league-wide general analysis. I've used a lot of words such as typically, on average, and expected. For individual teams, there are a lot of other variables, the most significant of which is 1st down success rate--65% is only the league average. For example, the 2006 Colts' 1st down success rate was 79%. In contrast, the Buccaneers' success rate last year was only 59%. That's going to have a stronger effect on the probability of scoring than starting field position. But those 4 yds still matter a good deal.

Post Script--I asked KC Joyner about this topic. He pointed out that by "meaningless," he was referring to the statistic of starting field position due to its lack of context, and not field position itself.

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6 Responses to “The Importance of Field Position”

  1. Tarr says:

    One way to think of those 4 extra yards is that they would typically require 0.4 more first downs to score.

    This seems inaccurate to me. It would only be true if all first downs gained exactly 10 yards. It would be more accurate, I think, to say you need 4/N more first downs, where N is the average yards per first down.

  2. Anonymous says:

    I'm confused that he would consider the "statistic" as meaningless. I guess it depends on what you mean as "meaningful" - you can use average starting field position to correct points/drive to get it closer to a measure of defense independent of offense.

    Doesn't work great, but it does work.

  3. Brian Burke says:

    I agree with you. But in his defense, only 4 yards separated the #2 and #32 teams in starting field position. Maybe one or two big kick returns might account for that difference. This might be one of those stats where median is more useful than mean.

  4. Anonymous says:

    Through the first 6 weeks of 2007, the teams that were top 3 in average starting field position were a combined 15-1. The bottom 3 were a combined 4-12.

    Do we really need a math equation to realize how huge field position is? I don't even need to argue, because the records prove it. It may only be 4 extra yards, but that team still wins.
    But here's a stat for you: teams that have the highest average yards per pass win 60 percent of the time.

  5. Brian Burke says:

    Going through some old comments. I thought I should address this one.

    1. The N in Tarr's comment should be about 15-17 yds depending on how you count first downs. TDs are technically first downs, but they are truncated by the goal line, sometimes worth at most 1 yd. I agree with his analysis.

    2. The Nov 23 anonymous comment confuses correlation and causation. Winning teams have good offenses and defenses, which themselves cause field position. For example, a good offense will move the ball into opponent territory even when the drive doesn't produce any points, leaving the weaker opponent with poor field position. Vice versa for defense.

    What I'm trying to figure out is how the distance to the goal line affects the probability of scoring for an offense (or probability of stopping a score by a defense.)

  6. Anonymous says:

    onside kick every time
    number of possessions are more important than field position unless you are protecting a lead late in the game.

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