It's a desirable commodity in the baseball world.

It's a big reason the New York Mets committed four years and $60 million to Curtis Granderson, and a big reason players like Ike Davis continue to get chance after chance to find themselves in MLB.

Power, or even just the potential to hit for power, is valuable. It will never go out of style in baseball.

When most people think of measuring power, the first stat that comes to mind is probably slugging percentage.

Slugging percentage, or SLG, is simply a measurement of total bases per at-bat.

It's a pretty common rate stat that does a solid job measuring how often a player hits for extra-bases, for the most part. It's not a perfect stat, but it's a solid stat to use as a general baseline for power production.

The statistic I want to introduce today, however, is a little more refined.

Isolated power, or ISO, looks to measure a hitter's raw power. Unlike SLG, which can be skewed by, say, a player with a high batting average that hits a bunch of singles, ISO aims to take those singles out of the equation.

ISO is calculated by simply subtracting the batting average from the slugging percentage. SLG — AVG = ISO

On the surface, you might wonder why ISO is useful. We already have SLG, why take AVG out of the equation? Like my recent piece on BABIP, it's about trying to measure something as accurately as possible.

If the goal is measuring raw extra-base power, singles aren't a terribly relevant aspect of the equation. Since AVG measures all base hits as equal, subtracting it from SLG gives us the isolated power, the extra-bases per at-bat.

Fangraphs has a nice primer on ISO in their glossary, including a handy chart with assorted ISO figures that serve as a rough estimate for what qualifies as a good/bad/decent ISO. The league average varies each year, of course.

SLG is a good stat. The problem is, without further context, SLG can be somewhat narrow and misleading.

Real world example:

In 2013, Michael Cuddyer of the Rockies and Brandon Moss of the Athletics were ninth and 10th, respectively, in slugging percentage among all qualified hitters. Cuddyer slugged .530, while Moss slugged .522.

Cuddyer had a .919 OPS, while Moss had an .859 OPS. But Moss hit for more power than Cuddyer did, despite SLG and OPS showing otherwise. Moss outhomered Cuddyer 30 to 20, and ISO tells the story.

Moss had an isolated power of .267, compared to Cuddyer's .198. In fact, the only players in the Major Leagues who had a higher ISO than Brandon Moss last year were Chris Davis and Miguel Cabrera, amazingly.

Even though Moss hit just .256 on the season, he made those hits count. His .267 ISO shows the raw power he had, as he was able to put up a top 10 SLG in MLB last season despite a pretty pedestrian batting average.

Meanwhile, Cuddyer won the NL batting title with a .331 AVG, which is why his ISO was much, much lower than the ISO Moss put up, despite a slightly higher SLG overall. Not all slugging percentages are created equal.

What I like about ISO is that it's not all that complicated, as far as statistics go. If you can handle AVG, SLG, and OPS, you can get a pretty good grasp on ISO. It's a useful stat that I think will become more common in player evaluation in the coming weeks and months. It's nothing revolutionary, but it goes a long way towards dissecting whether or not a given slugging percentage is aided by a bunch of singles, or due to a legitimate show of power.


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