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Measuring a hitter's raw power with Power Factor

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Isolated power (ISO) is a great stat, but it is limited. Power Factor both sounds cooler and is a better measure of a hitter's raw power.

Rick Osentoski-USA TODAY Sports

There once was this dude named Wily Mo Pena. Wily Mo Pena did not play defense very well, he didn't hit for a very high average, and his plate discipline was pretty terrible. Fortunately, Wily had one skill that worked in his favor, and he used it to carve out an eight-year career in the big leagues: power. A microcosm of this was his 2011 season when he hit .205 with a .250 on-base percentage, but slugged a solid .416. His isolated power was .211, indicating a powerful slugger.

Fortunately for all, this is not an article about Wily Mo Pena. This is an article about the flaws of isolated power, or ISO. Wily Mo was the guy mentioned in the lede because when I think of the term "raw power" I think of him hitting one of the longest home runs in Comerica Park history. He's the poster child of this issue with ISO: hitters with lower batting averages rate out better than those with high batting averages.

ISO is the next in a long line of ways people have tried to measure slugging ability. Once upon a time, people counted home runs to see who was the best. Then the day came where someone said, "Wait, maybe we should consider doubles and triples as well!" giving rise to slugging percentage. It turned out slugging percentage had its flaws as well, namely that a guy who hit a bunch of singles could have a higher slugging percentage than a homer-or-bust slugger.

So recently, ISO has come into vogue in the sabermetric circles. The stat is calculated by subtracting batting average from slugging percentage to better isolate those who have more extra-base ability than others. To put it into example, if Player A hits .250 and slugs .400, he will have an ISO of .150. This would indicate more power than Player B, who hits .325 and slugs .450 for an ISO of .125.

Now let's consider another hypothetical. If Victor Martinez smacked ten doubles in his next ten at-bats, he would be hitting 1.000 with a 2.000 slugging percentage and a 1.000 ISO. If J.D. Martinez only had three hits in his next ten at-bats, but they were all home runs, he would be hitting .300 with a 1.200 slugging percentage and a .900 ISO. Per this scenario, Victor would have the higher ISO and be the better slugger. However, J.D. is the one hitting for more raw power. Therein lies the problem with ISO: by using subtraction to compare slugging percentage and batting average, the cumulative effect of hits is still evident. Power is not a cumulative thing. Power is about how many bases one can get in a single swing.

With that in mind, an alternative measure to extra-base hitting is to divide slugging percentage by batting average. This metric would tell how many bases a batter gets per hit. The official name for this statistic is power factor (PF), and it has been around for a few years. It hasn't gained nearly the traction that ISO has, however.

In our example above, Victor Martinez's PF is 2.000: he gets 2.0 bases for every successful base hit. The PF of J.D. Martinez is 4.000, a perfect score for homering every hit. By measuring the bases per hit, we can better see who packs the most punch.

Here is a list of Tigers hitters with a at least 40 plate appearances, showing their respective slugging percentages, ISOs, and PFs on the year.

J.D. Martinez 0.572 0.281 1.97
Collins 0.49 0.216 1.78
Cespedes 0.488 0.197 1.68
Cabrera 0.578 0.227 1.65
Avila 0.342 0.132 1.62
Davis 0.432 0.158 1.58
McCann 0.426 0.153 1.56
Castellanos 0.372 0.128 1.52
Holaday 0.396 0.125 1.46
Gose 0.382 0.106 1.38
Victor Martinez 0.379 0.105 1.38
Romine 0.418 0.114 1.38
Kinsler 0.352 0.088 1.33
Iglesias 0.386 0.062 1.19

If there's any takeaway from that, it's that J.D. Martinez is a strong fellow and Jose Iglesias really isn't. They're both All-Stars, though, so having power isn't the only thing in baseball.

Here are the top 10 qualified hitters in baseball by PF as well as their rank by ISO.

Player PF PF rank ISO rank
Giancarlo Stanton 2.29 1st 2nd
Mark Teixiera 2.24 2nd 5th
Luis Valbuena 2.16 3rd 17th
Joc Pederson 2.13 4th 10th
Albert Pujols 2.11 5th 7th
Todd Frazier 2.1 6th 4th
Stephen Drew 2.09 7th 45th
Jose Bautista 2.09 8th 11th
Nolan Arenado 2.09 9th 3rd
Bryce Harper 2.07 10th 1st

It will come as no surprise to hear that Stephen Drew, who ISO and PF vastly differ on in calculation, has the lowest batting average of any of the top 10 power factor-rated hitters by far at .179. He also has a negative fWAR. Baseball can be a funny game.

The point of measuring raw power is not necessarily to measure the value a player provides. In fact, a lot of times sluggers like Wily Mo Pena find themselves unable to find continuous employment in Major League Baseball. However, it's been said that chicks dig the long ball, and measuring raw power lets us see who truly is the best at it.