# 2015 Expected ISO (xISO) in Review

Back in May, I introduced an equation that would calculate expected isolated power (thus, “xISO”) numbers for hitters based on their batted ball profile. The idea was to generate an equation that could accurately describe for how much power a hitter should be hitting based entirely on publicly available data (provided to FanGraphs by Baseball Info Solutions), as opposed to proprietary data, so all fantasy baseball enthusiasts could use it.

I won’t get into the nitty gritty again — you can click on the link in the first sentence if you want to open that can of worms — but I will provide the equation again for posterity:
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xISO = –.1396 + .1814*Pull% + .5136*Hard% + .2344*FB%

I’ll provide a table of xISOs for all qualified hitters below and deliver some insight regarding potential buy-lows and sell-highs.

Let me be clear, however, that xISO is more descriptive than it is predictive. As aforementioned, it describes what should have happened, but it does not necessarily dictate what should happen going forward. With that said, it does seem to be have predictive qualities: the correlation between first-half xISO and end-of-season ISO for 128 qualified hitters in 2015 is really strong (R = .7843) — almost identical, in fact, to the correlation for the fundamental equation above. I think this is largely a testament to how quickly batted ball rates stabilize and, thus, how reliable they can be in small(er) samples. Still, this relationship between an early-season and full-season samples prove promising for xISO’s ability to anticipate regression to the mean.

Before we go much further, though, let’s set the record straight:

1. Not all fly balls are created equal.
2. Not all pulled balls are created equal.
3. Not all hard-hit balls are created equal.
4. The data available to us does not discern how these three categories of balls in play overlap.

Thus, two hitters who may happen to have identical FB%, Pull% and Hard% may not deserve to have identical xISOs, despite what the equation may proclaim. In short, the BIS data are generalized enough to obscure the nuances in batted ball profile from one hitter to the next. It’s the easiest explanation why, although the xISO equation can be a helpful tool, you can’t expect live and die by it.

With that said, here’s a table of ISOs, xISOs and batted ball information of all 2015 qualified hitters, as promised. Overachievers and their varying degrees of overachievement are highlighted in shades of red; underachievers, in blue.

### OK, so, sell high?

Talking about selling high on someone is always relative, and it’s usually contingent almost entirely on the buyer’s perception of the player. Is Bryce Harper a sell-high guy? If you buy that he’s a generational, perhaps otherworldly talent, then no. But if you find someone who thinks he’ll hit 50 home runs next year? Sure, sell. Although whatever Harper nets you would probably have to be a monstrous haul for you to ship him, given the price point at which you probably obtained him in your 2015 draft.

No, it’s harder to find absolute sell-high guys, but I think I can still name a few.

Carlos Gonzalez, COL OF

After perhaps his most miserable start to a season in his career, he managed to not only stay healthy for once in his danged life but also break his previous career mark for home runs in a season. I took a lot of flak for an early call to sell based on his batted ball profile; it eventually improved, and he obviously rewarded the owners who bought low or remained patient. But his xISO thinks his monstrous second half was an overcorrection — an aberration* in light of his peripherals. Given the relevative mediocrity of the rest of his game and his injury propensity, CarGo is easily the most obvious sell-high name on the list.

Yoenis Cespedes, FA OF

His absurd second half will inflate his draft value, but his value also depends on with which team he signs. Yo’s a monster, but he obviously can’t keep pace with how he rewarded the Mets this year. At the same time, I think we’ve seen enough of him to know with a high degree of certainty what we can expect from him year in and year out. Still, if there’s an owner who thinks Cespedes has sudden “found it,” go ahead and flip him.

After an anemic power display in 2014, Eaton flashed the power-speed combo that made him my favorite Mac lookalike in baseball. Like CarGo, however, the slugging may have been an overcorrection to what could be chalked up as bad luck previously. An xISO differential of .035 is equivalent to a little more than five additional home runs across 600 PAs; take those away and Eaton merely flirts with double digits in a full season. (Of course, it’s not that simple, but is anything really that simple?)

Like I said before, though, anyone can be a “sell high.” It’s all relative. Dangle your bait and see what bites.

Right, buy-lows. I won’t get too formal with this one, but Brandon Moss could bounce back in a big way next year should he see regular playing time for the Cardinals. Troy Tulowitzki disappointed everywhere he went in 2015, but as xISO shows, it may not be entirely his fault.

More intriguing names include Michael Taylor, the four-tool prospect taking over center field for the Nationals in the wake of Denard Span. His desirable power-speed combination and less-desirable poor contact skills makes him almost a carbon copy of Steven Souza, except with much more upside. Indeed, the batted ball profile points up, and he could be an interesting late-round play that pays dividends, even if he may — nay, will — hurt your batting average.

Gregory Polanco shows he’s a prospect who may also have more juice in the tank. Frankly, he’s overscrutinized, and it’s possible he takes a big step forward next year should he maintain his decent batted ball profile and fortune falls in his favor.

I anticipate that, because I calculated xISOs for only qualified hitters, I’ll receive requests for xISOs of non-qualified hitters. I ask that you suppress this urge to make such requests and, instead, actually use the equation I have provided. However, I will oblige the lazy and inconsiderate — just let me have this minor victory of first calling you lazy and inconsiderate. You know I still love you!

Currently investigating the relationship between pitcher effectiveness and beard density. Two-time FSWA award winner, including 2018 Baseball Writer of the Year, and 5-time award finalist. Featured in Lindy's Sports' Fantasy Baseball magazine (2018, 2019). Tout Wars competitor. Biased toward a nicely rolled baseball pant.

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What do you think about the fact many guys, like Nelson Cruz, seem to trend as over/under achievers year after year?