Sunday, November 11, 2007

Steals, errors and speedy batters

Last time around, I commented on the similarities between the IBL's stolen base leaders and the leaders in reaching base on error. I glibly wrote that "Not surprisingly, baserunning ability is a key factor in the ability to reach base on error."

Is that really so? Let's plot the error rate (reaches on error per at-bat) versus the steal rate (steals per times on base) for the 50 players with at least 80 plate appearances in 2007:

Wow! What a strong correlation! (Note sarcasm.) Not much support for the theory, it seems. Except for a few outliers, most of the players are scattered in a random cloud with no apparent structure.

But if we massage the data just the right way...

Here's the same data, but restricted to the 20 players with the highest reach-on-error rates:

That's better, isn't it? In fact, we get a correlation of .77 and an R-squared of .59, both indicating a high degree of correspondence between the two statistics - for this group of players.

Does this mean anything?

I think it might.

I would suggest that there are two kinds of hits. There's a "power" hit, a solid base hit into the outfield gap, generally either a line drive or a sharp grounder. It's hit far out and between the fielders so that any batter can make it safely to first base.

Then there's the "speed" hit. Maybe it isn't hit as far, or it's not as hard for the fielders to get to. Whether or not it becomes a hit depends on the speed of the batter (and possibly other runners). If he can leg it out to first, he's got the hit. Otherwise, he's out.

A reach-on-error is predominantly a "speed" hit. It is predominantly a ground ball, but one which was (apparently) played poorly by the defense. If despite the error the batter is put out, the play is recorded as a normal out - no defense error is recorded. If the batter beats out the throw, however, he has "reached on error" - as if it was not his own skill and speed that got him on base instead of out.

So faster runners are more likely to reach base on error. There is no need to assume (as I erroneously wrote earlier, and as Tom Tippett seems to imply here) that some batters actually have the propensity to cause the defense to commit more fielding errors by hitting the ball in a hard-to-field way. Rather, the actual error rate may be distributed evenly among batters, but only some of them can consistently exploit defensive errors to get to first base. The others just get out, and no error is recorded.

Furthermore, it stands to reason that some error plays - especially at lower levels of play - are so egregious that even the slowest batter can reach base. This is clearly the case for wild throws, for example. So there should be a "background rate" of reaches on error that affects all batters, however fast or slow they are. This may explain the lack of correlation between steal rate and error rate for the bottom of the pack - all we're seeing there is random noise, not batter skill. Also, the sample size is probably too small to be significant, with just 1-3 reaches on error per hitter on the season.

Finally, if you haven't yet, please see Tom Tango's comments on an earlier post about steals and error rates. The salient point is that speed probably peaks at a younger age than strength (i.e., fielder's throwing ability). I would add to that that fielding skill, unlike running speed, is learned through experience and probably also peaks later. So the younger age in lower-level leagues can be expected to produce more steals and errors.

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