Wednesday, November 6, 2013

Why Slugging Percentage Matters

If You Don't Want to Read the Whole Thing: I assume you know what batting average is. Here, I showed that on base percentage has more of an impact on run scoring than does batting average. In this post, I show that slugging percentage is even more predictive. A player's slash line (batting average/on base percentage/slugging percentage, like .280/.350/.445) is an easy way to display all three figures.

But If You Do: I've already written about on base percentage and why it's important, more important that batting average. I'm going to take the same approach in looking at slugging percentage.

Slugging percentage has been around for a long time. I remember reading a piece in a SABR publication about 30 years ago that referred to an article from around 1915 or so. The author of the older article was trying to make a point about Gavvy Cravath, a slugger (given the context of the time) for the Phillies*. He said that batting average is like asking how much money someone has in his or her pocket, and getting a response of "nine coins." It doesn't tell what the coins are worth. Slugging percentage gives you the dollar value. Batting average is hits divided by at-bats. Slugging percentage is total bases divided by at-bats. To calculate total bases, a single's worth one, a double's worth two, a triple's worth three, and a home run's worth four. A triple might not be worth three times as much as single, as batting average suggests, but it's certainly worth more than a single. The major league leaders in slugging percentage last year were Miguel Cabrera (.636), Chris Davis (.634), David Ortiz (.564), Mike Trout (.557), and Paul Goldschmidt (.551). The major league average was .396.

The difference between batting average and slugging percentage is pretty intuitive. Albert Pujols batted .258 in 391 at bats. Placido Polanco batted .260 in 377 at bats. Close, right? But Pujols hit 19 doubles and 17 homers. Polanco had 13 doubles and a homer. That gives Pujols a .437 slugging percentage compared to .302 for Polanco, illustrating who's the better hitter.

How important is slugging percentage? As I did with on base percentage, I'll check the correlation with runs scored. The higher the correlation, the more more slugging percentage is tied to scoring, which is the object of offense. As I pointed out, the correlation between batting average (BA) and runs, using data from every team since 1995, is 0.80, which is pretty good. The correlation between on base percentage (OBP) and runs is better, 0.88. The correlation between slugging percentage (SLG) and runs is higher still, 0.90. Roughly speaking, this means that BA explains about 65% of run scoring, OBP 78%, and SLG 81%.

When talking about OBP, I noted that last year 24 players batted .300 or better. Joey Votto's .491 SLG was 24th. So we'll say a .490 slugging percentage is like a .300 batting average, just as a .370 on base percentage is like hitting .300.

Enter the Slash Line

I hope I've made it clear why on base percentage and slugging percentage are more important measures than batting average. But this doesn't mean batting average is unimportant. A 0.80 correlation's a good one. So analysts talk about a batter's slash line, defined as BA/OBP/SLG. (The slash separates the figures.) Miguel Cabrera's slash line was .348/.442/.636. That's insanely good, as expected, as Cabrera led the majors in all three categories. The major league average slash line last year was .253/.318/.396. 

You can tell a lot from a slash line. Cleveland's catcher/DH/1B Carlos Santana, .268/.377/.455: Great at drawing walks, decent pop, doesn't get a ton of base hits. (Players with a low BA and high OBP usually strike out a lot, as did Santana.) White Sox shortstop Alexei Ramirez, .284/.313/.380: A lot of his value is in his batting average, i.e., he doesn't walk a lot or have a lot of power, given his below-average on base and slugging percentages. 

I'm going to use slash lines going forward (starting with the footnote below). If you forget what they are, click on the Glossary at the top of the page.

*I've seen Cravath listed as the player whose career home run record Babe Ruth topped. That's not true. Ruth hit his 139th career home run in 1921, at which time he vaulted past Roger Connor, a first baseman, primarily for the New York Giants, who hit 138 homers during a 18-year career from 1880 to 1897. Cravath was the 20th century record holder, with 119 between 1908 and 1920. He hit all but two of his home runs for the Phillies, who played at Baker Bowl, a hitter's park that was only 281 feet down the right field line, though with a Green Monster-esque 60 foot fence in right. Cravath, though, was a right-handed hitter. Baseball-reference has split data for Cravath only in the last five years of his career, past his prime, but during that time he hit .304/.392/.547 at home and .254/.351/.386 on the road--basically David Ortiz at home and something like the Cardinals' Jon Jay on the road. 

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