Here's a Thought: Mariano Rivera's Weird Walk Rate

Nathaniel StoltzSenior Analyst IAugust 3, 2009

When we talk about a pitcher's "control" statistically, one of the first things we often jump to is walk rate. Pitchers with better control walk fewer batters.

Or so we think.

Another way to measure "control" is by using plate discipline variables. These aren't "fancy stats" by any means, but they describe every aspect of a batter-pitcher interaction.

The plate discipline variables are:

Swing%--Percentage of pitches swung at.

O-Swing%--Percentage of pitches outside the zone swung at.

Z-Swing%--Percentage of pitches inside the zone swung at.

Contact%--Percentage of swings that make contact (O-Contact% is for outside the zone; Z-Contact% is for inside the zone).

Zone%--Percentage of pitches thrown by a pitcher that are in the zone. This does NOT mean "percentage of pitches that are strikes." It means the percentage of pitches that find the zone (according to the ultra-sensitive MLB cameras meant to specifically measure this), regardless of whether they're called balls or strikes.

F-Strike%--Percentage of first-pitch strikes.

Sorry for that long list, but it's necessary information.

Anyway, these variables give us a nice look at a pitcher.

The higher a pitcher's O-Swing%, the more he gets batters to chase pitches, and the more "deceptive" he and his pitches are.

The higher a pitcher's Contact%, the more "hittable" his pitches are.

Obviously, Zone% and F-Strike% are very important as well.

At first glance, it may appear that Zone% is similar to walk rate: the more a pitcher pitches in the zone, the fewer walks he'll allow, right?

Well, sort of.

There's something of a correlation between the two, but it's not as strong as one would expect.

Just to use an extreme example, Jamie Moyer only finds the strike zone about 44 percent of the time, but he walks just about two batters per nine innings.

The Zone% and walk rate are both very low. Moyer doesn't throw strikes, but rarely walks hitters.

There's an easy explanation, however.

For one thing, batters chase a lot of Moyer's pitches. His 29.2 O-Swing% is well above the average.

Also, Moyer throws a bunch of junk to the plate, so when batters swing, they make contact and don't walk. Moyer's Contact% is 86.5, well above the MLB average. 

Moyer also throws a lot of first-pitch strikes (61.7%), so he gets ahead in the count and then throws the ball out of the zone.

So, while Moyer doesn't throw many pitches in the zone, he does three things to counteract that and lower his walk rate:

1.) Get batters to chase out-of-the-zone pitches, turning balls into strikes
2.) Inducing contact when batters do swing
3.) Getting ahead in the count

Those three things combine with Zone% to be the four important factors of walk rate.

Since Moyer's 3-for-4 on the factors, his walk rate is good.

However, while the four factors correlate to walk rate in most cases, occasionally, they don't.

There's no better example of this than Yankees closer Mariano Rivera.

Let's run through Rivera's performance in the four factors.

Rivera's pitches have hit the strike zone just 46.3% of the time, well below the average of 49.3%.

He does a decent job of throwing first-pitch strikes, coming in just above the MLB average in F-Strike%.

Batters do chase a lot of Rivera's pitches outside the zone (35.4%), and make a slightly-above-average amount of contact (83.3%).

So Rivera throws far fewer pitches in the zone than normal and rates slightly above average in the other three factors.

One would think Rivera has an average or maybe slightly better than average walk rate, right?

He's walked just 1.04 batters per nine innings.

Now, I'm sure most of you are thinking, "Well, Rivera's special and awesome, so he makes it work."

And I would totally agree with that, if it wasn't for the fact that he's never pitched like this before.

Consider Rivera's four-factor stats going back to 2002:

Year      O-Swing%      Contact%      Zone%      F-Strike%
2002      23.3              78.6             58.5          63.1
2003      29.9              75.2             55.0          60.7
2004      27.3              80.1             57.4          64.9
2005      26.0              80.0             52.8          61.1
2006      34.4              81.1             54.1          62.5
2007      35.4              76.6             53.4          60.7
2008      36.3              76.3             51.4          62.6
2009      35.4              83.3             46.3          59.4

Rivera's 2009 Zone% and F-Strike% are career-lows (at least since 2002, which is when pitch-by-pitch data began to be collected). The Zone%, especially, is far below any other one Rivera's put up.

His O-Swing% has basically held steady for four years as well.

Rivera's been slightly more hittable this year than in the past, but not enough to make up for the decrease in Zone% and F-Strike%.

Despite that, Rivera's walk rate is at the second-lowest mark of his career (2008 was the lowest).

If we look at 2007, just to pick an example, Rivera had the same O-Swing%, a much lower contact rate, a far higher Zone%, and a higher F-Strike% than 2009.

Yet Rivera's 2007 walk rate was 1.51 BB/9, higher than this year's 1.04 mark.

Rivera's numbers show that walk rate doesn't always reflect a pitcher's control properly. It's clear that he's thrown fewer strikes in 2009 than ever before, but his walk rate describes him as a pinpoint-control pitcher.

It will be interesting to see if Rivera's walk rate increases if his plate discipline numbers stay in this range.


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