Get to know a stat: What is ISO and what good is it?
One thing that can be daunting for the casual baseball fan is the preponderance of “new” statistics. The quotation marks are because some are legitimately new, and others—such as the one we’re looking at today—have been around for a very long time, but never made it to the back of baseball cards, so they’re less familiar.
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Today’s stat is a pretty simple one: we’re looking at ISO.
ISO is short for Isolated power, and while the term was coined by Bill James, the idea was first created by Allan Roth, who got his first job in baseball in 1947 working for Branch Rickey. (Roth’s story is fascinating, by the way. Jerome Holtzman often gets credit for “saves” becoming a stat, but a slightly different version of the stat was also one of Roth’s. You can read more about him here.)
But back to ISO. How is it calculated, and what knowledge can you glean from it?
For the first, it’s actually a pretty simple equation: (slugging average) – (batting average). That’s it!
For those of you who are still in the early stages of learning statistics, let’s break those two ingredients down a little. Slugging average (also sometimes referred to as “slugging percentage”) tells you how likely a batter is to hit for extra bases. Whereas batting average is: # of hits / # of at-bats (so for instance, 6 hits in 20 at-bats would be 6/20, or a .300 average), slugging tells you a little more about those hits. Doubles count twice, triples count three times, and home runs count four times.
Let’s say we have two players, and they’re both 6-for-20. Batter A has three singles, a double, and two home runs, so his slugging percentage would be ((3*1)+(1*2)+(2*4))/20, or as it would be written on a stat sheet: .650. Batter B, on the other hand, had five singles and just one double. That’s ((5*1)+(1*2))/20, or .350. So if your job was to evaluate hitters, you could look at two .300 hitters and definitively say the first one provided more offensive value.
Back to ISO: In the scenario above, subtracting batting average from slugging average would give Batter A an ISO of .400, and leave Batter B with an ISO of just .050.
So why do we need ISO when we already have slugging percentage? Well, you don’t need it, but it can simplify a lot for you. Let’s take two new players:
Player A’s SLG is .471
Player B’s SLG is .532
Player C’s SLG is .537
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If that’s all you have to work with, you might assume that all three players have similar power. To get a better feel for it, you have to include batting average. Here are those:
Player A’s BA is: .297
Player B’s BA is: .312
Player C’s BA is: .225
Now you have a better feel for what’s happening. Player A’s slugging is being boosted by a high batting average, and player C’s overcomes a low one.
This can be simplified by looking at ISO:
Player A (Elvis Andrus in 2017) has an ISO of .174
Player B (Adrian Beltre in 2017) has an ISO of .221
Player C (Joey Gallo in 2017) has an ISO of .327
In short, all three had great years at the plate, but ISO gives you another indicator of how they did it. Elvis got a lot of doubles, and hit nearly .300, Beltre had an incredibly-balanced year, and Gallo spent the season trying to send souvenirs to God.
League average for ISO is usually somewhere around .140, and if you start seeing numbers in the .200s, you know you’re looking at a genuine power hitter. That bears out here: While Andrus had a career year in 2017, he’s not a true “power hitter”, having ISO’d under .100 for seven of his first eight seasons in the league. Beltre is a legitimate slugger, and Gallo is an otherworldly monster who subsists solely on the slumped shoulders and whiplashed necks of pitchers.
It usually takes about as long for ISO to normalize as it does batting average (obviously, since it is built of AVG and SLG), so it’s probably too early to suggest that, say Mookie Betts will keep up his league-leading .480 ISO—Betts has put up ISOs of .188, .216, and .194 in his three years in the league—but it does give some insight into just how crazy his 2018 season has been.
Gallo, for what it’s worth, is currently 16th in the league with an ISO of .276. The first number will come down, the second will rise.
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