Isolated Power Stands Strong, but It Can Still Fall Short

If you’ve watched The Brady Bunch, Family Ties, Community, or pretty much any other sitcom, I’m sure you’re familiar with the “two dates to the dance” trope. The premise is exactly what it sounds like, and antics are guaranteed to ensue. It almost always ends in disaster, and the wannabe Lothario learns their lesson. If they had only picked a single date, they might have had a lovely evening. Instead, as Confucius says, “The man who chases two rabbits catches neither.”

It’s not just TV characters who try to pull this off; some of the most prevalent baseball statistics are guilty of double dating, too. In particular, I’m talking about the stats that try to court the analytics crowd and more traditionally-minded fans at the same time. This is an admirable endeavor (unlike two-timing your prom date), but that doesn’t make it any less of a fool’s errand.

OPS+ is the perfect example. It takes a widely understood statistic and revamps it for the modern age, but as a result, it combines all the inaccuracies of OPS with all the complexities of park and league adjustments. It’s too much for most casual fans to wrap their heads around, yet it still undervalues on-base percentage and overvalues extra-base hits — cardinal sins for the hardcore sabermetricians among us. I’ve long thought that isolated power falls in the same category. It’s missing the simplicity and storytelling quality of batting average and total bases, but it also lacks the precision of advanced numbers like wOBA and wRC+. Thus, I’ve never fully understood who the target audience for ISO really is.

The formula for slugging percentage is total bases divided by at-bats. This means a double carries twice as much weight as a single and half as much as a home run. In reality, a homer isn’t four times more likely to bring in a run than a single, but at least there’s a method to the madness: a single gets you one base, a double gets you two, and so on. Isolated power employs a similar logic, but it’s a little less straightforward. ISO only counts extra bases, so a double is worth one, a triple worth two, and a home run worth three. Essentially, ISO makes singles the baseline and only gives batters credit for the additional bases they reach. The formula looks like this:

Like slugging percentage, ISO is technically a weighted statistic, but as the FanGraphs glossary explains, “it counts extra bases, but it does not properly weigh their importance in a value sense.” This means it estimates the value of each extra-base hit by counting bases rather than calculating changes in run expectancy, and therein lies my concern. We already have the necessary data to develop a more accurate version of ISO, so why don’t we do it?

Well, amusingly enough, it turns out the blunt and rudimentary method works nearly as well as the advanced approach. You can find the weighted values we use to calculate wOBA on the FanGraphs Guts! page; as of right now, the weighted value of a single in 2023 is .885. If you subtract that number from the weighted values of a double, a triple, and a home run, you’ll end up with .363, .690, and 1.128, respectively. According to those calculations, the added value of a triple is basically twice the added value of a double, and the added value of a long ball is approximately three times that of a two-bagger. In other words, isolated power takes the wrong route but arrives at essentially the same destination nonetheless.

I still think ISO inhabits a strange middle ground between traditional stats and sabermetrics, but after discovering how accurate it actually is, I’m not nearly as concerned about that. If it were entirely up to me, I’d prefer a stat that weighs each input as precisely as possible, but I do see the appeal of a metric that gets the point across with math you can do in your head. Using ISO is like going to New York on a flight with layovers in Boston, Paris, and Tokyo before eventually arriving at the Newark airport. The flight path is seriously flawed, and the destination isn’t perfect, but ultimately, it gets you where you want to go for half the price.

There’s another way that ISO falls short, though. ISO separates the value a hitter accrues via power hitting from the value he accrues by reaching base; that’s why it’s called isolated power in the first place. But if I’m looking to evaluate a hitter’s power, I want to know how much total value he’s producing with his extra-base hits, not just how much extra value he’s adding on top of a theoretical single. After all, reaching base is an essential part of hitting for power, and hitting for power is a great way to reach base. It’s not as if extra-base hits are just singles with a bit more power, or all singles are failed extra-base hits. The best power hitters are those who contribute with doubles, triples, and home runs; conversely, the worst power hitters are those who don’t.

Therefore, I think there’s room for a weighted version of ISO after all, a stat that communicates how much value a hitter produces with his extra-base hits. To create this, I took the values of a double, a triple, and a home run from our Guts! page and plugged those into the formula for isolated power. It’s not quite as “isolated” as ISO, but it still separates extra-base hits from every other outcome of a plate appearance. Here’s the formula for wISO (for lack of a better name) as of July 18:

One could say this metric is too similar to wOBA, and indeed, the two are strongly correlated. It does, after all, have three of the same inputs. But by the same token, you could accuse traditional ISO of being a dolled-up version of home run rate, since the formula for ISO values home runs so heavily. Indeed, among qualified hitters this season, there’s almost a perfect positive correlation between ISO and HR/PA. I’d argue that’s another reason why wISO is worth our attention.

I ran the wISO numbers from 2022 first and was immediately pleased with the results. In the column on the left, the three players I cherry-picked are ranked by ISO. In the column on the right, they’re ranked by wISO. I don’t know about you, but I certainly think the second column is a more accurate power ranking, so to speak:

Moving on to the season at hand, these are the top 20 qualified hitters in 2023 by wISO:

I turned wISO into a “+” stat to make it easier to compare with regular ISO; otherwise, the two are on totally different scales. I’ve also taken note of how each player’s league-wide ranking changed from one leaderboard to the other. Some players shot up (or fell down) the leaderboard, but their ISO didn’t really change compared to league average. In contrast, others saw a big change in their ISO relative to league average, but their ranking remained relatively stable. For example, Shohei Ohtani’s wISO is significantly lower than his regular ISO, but that doesn’t change the fact that he’s the best power hitter in the game.

On the flip side, these are the bottom 20 hitters by wISO:

None of these guys have much power according to either metric, but most have a little more pop than their ISO would have you believe. In particular, you might notice a lot of speedy contact hitters on the list. These guys rarely hit one out of the park, but doubles and triples are extra-base hits, too, even if you earn them with your legs.

Next up, these are the top players by the difference between their wISO and regular ISO:

For most of these guys, the difference is negative, which makes sense. It’s easy to skew your ISO with a ton of home runs, like Ohtani and Pete Alonso have done, but it’s harder to skew it in the opposite direction. You can’t hit fewer than zero home runs (and believe me, Myles Straw has tried), and even the slappiest of slap hitters gets a homer every now and then.

Since almost all of those hitters had a negative ISO differential, here are the top hitters with a positive difference between their wISO and regular ISO:

These are the guys that I think isolated power is underrating the most. Alex Verdugo is a particularly notable inclusion; by ISO he’s barely above league average, but wISO gives him more credit for his 27 doubles, tied for second in the AL and third in baseball.

Next, these are the hitters who saw their league-wide rank change the most in either direction:

Atop the list is Randy Arozarena, essentially the mirror image of Verdugo. ISO would have you think he’s an above-average power hitter, thanks to his 16 home runs, but he only has nine doubles, tied for last among qualified AL batters. Another interesting name is Marcell Ozuna, whose home runs are doing even more heavy lifting than Arozarena’s; he has 17 of them but only eight doubles and nary a triple to speak of.

Finally, these are a few additional hitters worth spotlighting. Anthony Volpe and Michael Conforto drop from above average to below average by wISO; Nathaniel Lowe and Carlos Santana move in the opposite direction:

Ultimately, wISO shouldn’t transform your opinions about any individual batter; it’s not a revolutionary new stat by any means. At the same time, it’s different enough from regular ISO to be worth considering, and it provides some additional information that ISO does not. Most importantly, it isn’t so heavily skewed by home runs, so it highlights how much a batter produces with all his extra-base hits.

I feel more confident in ISO than when I started writing, and I must admit that it holds up better than I thought. Still, that doesn’t mean we couldn’t benefit from a new metric for evaluating power. The best power hitters use their big bats to help them reach base, and that’s a skill that shouldn’t be ignored.

Stats as of July 18.