When it comes to minor league players, FanGraphs does a solid job of providing standard and advanced stats. Visit their Minor League Leaders page, and you’ll find pages of hitting numbers for players at every level of the minors.
Unfortunately, while the numbers are easy enough to find, they’re not as easy to draw conclusions from. These days, we’re used to numbers being contextualized: adjustments for season and for park and everything in between. When it comes to minor leaguers, though, the numbers are served about as raw as a sashimi tuna roll.
For example, if you look at the advanced tab, you’ll find that Kevin Kramer posted a wRC+ of 141, good enough for third among qualified AAA players and ahead of Christin Stewart, who last season was ranked the Detroit Tigers’ number three prospect by FanGraphs. Not bad, right?
Well, let’s take a closer look and see how those numbers hold up.
The first adjustment we need to make is for park factor. (If you’re unfamiliar with park factors, FanGraphs has a good explanation.) Simply put, park factors are a way to adjust a hitter’s performance for the stadium in which they played. The calculation is simple, based purely on runs scored at home and runs scored on the road.
After pulling that data from MiLB’s website (not an easy task), I was able to come up with park factors for AAA and AA teams. (Note: for purposes of this post, we’ll stick to the International and Eastern Leagues, since that is where the Pirates AAA and AA affiliates play.)
|Lehigh Valley IronPigs||PHI||1.08||1.04|
|Toledo Mud Hens||DET||0.95||0.97|
|Pawtucket Red Sox||BOS||1.00||1.00|
|Rochester Red Wings||MIN||1.00||1.00|
|New Hampshire Fisher Cats||TOR||0.98||0.99|
|Portland Sea Dogs||BOS||0.99||0.99|
|Reading Fightin Phils||PHI||1.07||1.03|
|Binghamton Rumble Ponies||NYM||0.95||0.97|
|Hartford Yard Goats||COL||1.09||1.05|
|Richmond Flying Squirrels||SFG||0.87||0.93|
The raw single-season park factor is the first number. Since this is only one year of data, however, and because these things are prone to variance, I’ve also regressed 50% back to the mean of 1.00. I’ll be using the regressed numbers for all calculations.
Now, with park factor taken into account, we can adjust the wRC+ numbers. Here are the numbers for all Pirates AAA hitters with at least 200 plate appearances last season. The adjusted wRC+ is on the right.
I must note here that there’s some slight discrepancies between my original wRC+ numbers and FanGraphs’ numbers. Since they don’t publish their Minor League weights, I used their standard numbers and used an estimate based on a linear model to estimate the wOBA scale.
Good news! Kramer’s numbers look even better now. Giddy-up.
Now for AA hitters:
The Ke’Bryan Hayes numbers are particularly encouraging.
Let’s take our evaluation one step further. Here, again, are the AAA and AA numbers, but this time with one additional column: batting average on balls in play (BABIP).
As you can see, BABIP has a rather large influence on the wRC+ outcome. In fact, Kramer nearly hit .400 on balls in play last season! That likely plays a big part in his 142 wRC+.
So what happens to those numbers when we adjust for some measure of “luck”? By adjusting a player’s hits based on league-average BABIP, we can see just how much luck has inflated the raw numbers.
Last season, league-average BABIP in the Major Leagues was .296. In the International League, the average BABIP was .309, while average for the Eastern League was .307. It makes sense that those numbers would be higher for lower levels of baseball given defense and pitching talent discrepancies, though it’s hard to say why the AA level actually produced a lower BABIP number.
Now, after adjusting a player’s singles, doubles, and triples so that their BABIP falls in line with the league average, while maintaining the same ratio, we can recalculate wOBA, wRAA, and, of course, wRC+.
AAA Adjusted wRC+
|Name||PA||wRC+||PF wRC+||LUCK wRC+|
The luck-adjusted numbers for Kramer now look much more pedestrian. (In case you were wondering, Christin Stewart’s adjusted wRC+ is 147 after adjusting for park and BABIP luck.)
AA Adjusted wRC+
The Hayes numbers look less impressive now, but they offer a brighter outlook for Will Craig. Of course, it’s also worth noting in those comparisons that Craig is two years Hayes’ senior.
In short, while there is plenty of good data out there on baseball prospects, it’s always important to remember just how these numbers look when put into a wider context. A player’s park, as well as their luck, can play a large part in how good or bad they look. And while, over time, player’s may prove higher BABIP numbers are due more to skill than luck, it’s still worth keeping in mind when evaluating prospects.