Sabermetrics 101: The ISO Factor

Power has become a premium in baseball. The ability to hit for extra bases and home runs is now more coveted today than any other time in recent memory. And for years slugging percentage has been one of the primary indicators of a hitters power value. However, the metric is flawed. While slugging percentage, yes, may be used to gain valuable insight into a hitters production, it fails to make this simple yet distinct statement: all hits are not created equal. 

Sounds very non-American for America’s pastime, right?

However, there is another metric by which we can utilize that does treat extra-base hits (doubles, triples, and home runs) differently: ISO, otherwise known as Isolated Power.

But before you happily jump aboard the ISO bandwagon thinking it is superior to other hitting metrics, please consider this notion: ISO is a flawed metric as well. 

This now begs the question about why do us as baseball fans and analysts would consider using ISO if it is, in fact, flawed. The ISO metric does not hold significant predictive authority unless there is a sufficient data size (more than 550 plate appearances) nor is it park- or league-adjusted. Not to mention it doesn’t use accurate weight values for doubles, triples, and home runs, and doesn’t consider the situation surrounding circumstances of the extra base hit. However, despite it’s shortcomings, the goal of the ISO metric is to just provide a number associated with a hitter’s raw power, or in other terms, the rate at which they hit for extra bases. In this regard it does have a more accurate measure than slugging percentage.

But what is ISO exactly?

Isolated power, or ISO, is a calculation that informs how often a hitter’s result is a double, triple, or home run. Where as slugging percentage includes singles in it’s formula, ISO does not. The calculation also applies weights to triples and home runs, which is an improvement over the no weights approach applied to slugging percentage. There is also a sliding scale, courtesy of, in which you can reference to know what is considered an excellent or awful ISO.

Courtesy of Fangraphs

But enough talking about the calculation and the sliding scale, let me show it so you can use it for yourself.

ISO Comprehensive Formula: 

ISO = (2B + 2 x 3B + 3 x HR)/(AB) = SLG – AVG

As you can see, if you break the formula down, slugging percentage minus batting average will also suffice in place of the weights associated with triples and home runs. ISO is a fairly simple formula to comprehend and utilize in your own analysis. But there are some pitfalls that need to be discussed when utilizing ISO in hitter analysis and comparisons, such as, not all ISO are equal. Since SLG – AVG also equals ISO then that must be analyzed closer. For example, let us examine two players with identical ISO, but with varying slugging percentages and batting averages.

Ryan Howard, Philadelphia Phillies: .213 ISO, .238/.450

Jose Abreu, Chicago White Sox: .213 ISO, .292/.504

Both Howard and Abreu are arguably having different levels of success in 2015, even though they are each averaging the same rate of extra base hits according to their own respective ISO. It would be irresponsible though to conclude without further investigation about whom is the better hitter in 2015, no matter how obvious it may seem.

However, to reach the conclusion of Hitter A is better than Hitter B properly, then other metrics must be utilized that provide a more comprehensive indicator of hitter’s production besides ISO. In the end, this is all about forming a hypothesis and amassing evidence that will support your claims. Isolated power should only be a piece of the evidence gathered to help fill in the picture, other metrics must be used to figure out the rest.

**Statistics and information about Isolated Power via Fangraphs. 

Cody Poage is a contributor for Staliners. Follow him on Twitter @cpoage9. 

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