The “Back to the Basics” series is designed to explore the foundation principals of statistical analysis across the four major American sports. The series will provide readers with an understanding of how teams approach roster construction and why certain decisions are made both on and off the field. Readers will also be directed to additional information sources, such as websites, books, or even magazine articles that could substantially increase their knowledge of the subject at hand.
Isolated Power (ISO) is a statistical measure of the raw power of a hitter, essentially reflecting the number of extra bases generated per at-bat. First invented by Dodger owner Branch Rickey and Al Roth in the 1950’s, ISO allows us to quantify who the “power hitters” are in today’s game more accurately than the traditional statistic of Slugging Percentage (SLG). This is due to the fact that unlike SLG, ISO does not give credit to batters who hit singles.
For example, if a player were to hit 200 singles in 200 at-bats, he would have an ISO rating of 0. However, if the batter hit 200 home runs in 200 at-bats (unrealistic I know, but stick with me here), he would have an ISO rating of 3.0. Why 3.0? The batter would have generated 3 extra bases [2b, 3b, Home] for every 1 at-bat.
There are three distinct ways of calculating a player’s ISO rating. We'll take a look at the three different ways and provide examples for each method.
The first, as demonstrated above, is to total up the number of extra bases a player has produced and divide the total by the number of at-bats the player has accumulated. When calculating this total, be sure to remember the following values: Double [1 Extra Base], Triple [2 Extra Bases], and Home Run [3 Extra Bases]. Under this method, the formula looks as followed:
ISO = [(2B*1) + (3B*2) + (HR*3)] / At-Bats
Method 1 Example: Evan Longoria, 3rd Basemen [TB]
In 2013, Tampa Bay 3rd Basemen Evan Longoria posted the following statistics:
Now, for our calculation, some of this stuff is irrelevant for now, but more on that later. We know from earlier that ISO is merely the sum of the extra bases Longoria produced, divided by the number of at bats he had throughout the season. Let’s start by calculating the number of Extra Bases Longoria produced. We know Doubles (2B) are worth 1 point, Triples (3B) are worth 2 points, and Home Runs (HR) are worth 3 points. Therefore we know that Longoria produced 141 Extra Bases in 2013.
Extra Bases = (2b * 1) + (3b * 2) + (HR * 3)
Extra Bases = (39 * 1) + (3 * 2) + (32 * 3) = 39 + 6 + 96 = 141 Extra Bases
We also know that Longoria had 614 official at bats in 2013. Therefore, we can calculate Longoria’s ISO rating by dividing 141 by 614.
ISO = 141 Extra Bases / 614 At Bats
ISO = .2296 or .230
Based on the data, we can conclude that Evan Longoria had an Isolated Power rating of .230, well above the league average ISO of .143. Now, I mentioned above that there are two additional methods for calculating ISO. The second method is to take Total Bases, subtract the number of Hits, and divide the resulting difference by At Bats.
ISO = (Total Bases - Hits) / At-Bats
Method 2 Example: Evan Longoria, 3rd Basemen [TB]
Let’s continue to use Evan Longoria as an example. In 2013, Longoria produced 306 Total Bases, on 165 Hits, in 614 At Bats. After plugging in the numbers, we find this equation to yield the same result as the previous method. In 2013, Evan Longoria posted an ISO rating of .2296 or .230.
ISO = (306 Total Bases – 165 Hits) / 614 At Bats
ISO = 141 / 614 = .2296 or .230
The methods we just ran through will give you a more exact calculation, but if you want to ballpark a players ISO rating, simply take his Batting Average (AVG) and subtract it from his Slugging Percentage (SLG).
ISO = SLG - AVG
Method 3 Example: Evan Longoria, 3rd Basemen [TB]
Now, chances are you aren’t carrying around a calculator or are able to do a Method 1 or 2 calculation in your head (If you can, touché). Method 3 allows for a good approximation of a players ISO rating and can be done in a much quicker manner. Let’s use Mr. Longoria one more time as an example. We know from the data above that he had a Slugging Percentage of .498 and a Batting Average of .269.
ISO = .498 - .269 = .2290 or .229
Using Method 3 for our calculation, we can conclude that Longoria had a 2013 ISO rating of .229. As you can see, the rough estimate barely strays from the calculations found in Methods 1 and 2 [.230 vs .229], but it is far less labor intensive. However, all three methods can be used depending on your preference.
FanGraphs points out in its description of ISO that it takes a long time for the stat to have any predictive value. They recommend a minimum sample size of 550 plate appearances (note, this is Plate Appearances, not At-Bats, and therefore Walks, and Reaching Base by Error are to be taken into account). In the end, ISO is a great way of discovering value in players who may not put the ball in play often, but when they do, their hits are extremely impactful.
Quick Stat: In 2013, MLB league leaders in ISO with a minimum of 550 Plate Appearances were: (1) Chris Davis (.348); (2) Miguel Cabrera (.288); (3) Edwin Encarnacion (.262); (4) David Ortiz (.255); (5) Paul Goldschmidt (.249). Check out the full list here at FanGraphs.com.