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Statistical Prospect Evaluation Primer: Part 2: Hitters

Part 2 of series. You can read part 1 here.


The first thing I look at when evaluating a hitter is obviously his triple slash line (AVG/OBP/SLG), but I don't spend too much time dwelling on them. I apply the afformentioned context to the numbers and move on. There are better indicators of future major league success than a sky-high OPS, which can mask other underlying problems.

Strike Zone Control

There is nothing that can derail a prospect's ascent to the major leagues faster than poor strike zone control. Oakland has famously enforced a 10% walk rate rule for prospect advancement. A prospect must walk in approximately 10% of his plate appearances before they'll advance him a level. I don't think you should stick to such a hard-and-fast rule when evaluating a prospect -- there are always exceptions to the rule. Can you imagine leaving Alexei Ramirez or Vlad Guerrero behind because they didn't walk enough? -- but I do have a few marks I look for.

For the hitter portion of this project, I'm going to focus on Josh Fields.

Contact rate -- Fields has struck out in almost 27% of his AAA plate appearances, and not surprisingly over 31% of his major league PAs. His strikeout rate is simply too high to maintain a high average at the major league level, as we've witnessed first-hand with his inability to hit the average major league fastball. I'd argue that anything over 15% is entering the danger zone.

Walk rate -- Luckily, Fields has shown a a steady progression in his ability to draw walks (until his lost season last year). He went from drawing a walk in under 10% of his PAs in AA in '05 to over 15% in his brief repeat of AAA in '07. You can't walk your way to the majors though, and you need to weigh both K and BB rates when evaluating a prospect.

K/BB ratio -- Number 3 behind age and triple slash line on the list of first things I look at when scanning a hitting prospect's stats is K/BB ratio. It's arguably the most important measure of a prospect's development and future major league viability.

While a high K-rate or low BB-rate are troubling signs, they're not necessarily a kiss of death for a prospect. There should be a sliding scale for the K-rate or BB-rate you find acceptable in a prospect based on his other attributes. You expect power guys to strike out more, but you also want them to offset that low average potential with an increase in walk-rate. K/BB ratio helps put that in perspective.

Fields displayed troubling K/BB marks (over 4:1) in his first season of pro ball, but improved each year in the system. He posted a 2.6:1 mark in his first taste of AA, and 2.5:1 mark in his first full-season of AAA. He got his K/BB ratio under 2:1 in his first 200 at-bats before his call-up in '07, which, not coincidentaly, is exactly the point when I finally started to become a believer in Fields.

The 2:1 ratio as about as high as I'd accept from a top prospect, someone who you'd expect league-averagish production from in an eventual full-time role. Elite prospects, future stars, "can't-miss" guys should have a K/BB ratio closer to 1:1. Now, Chris Getz has a K:BB ratio near 1:1, but that doesn't mean he's an elite prospect. He lacks the power to put him in that category. In fact, I expect prospects who lack power to be around the magic 1:1 ratio. They can't survive at the major league level if they don't put the ball in play at a high rate, and are able to fight off pitches to extend at-bats and coax more than a few walks.

Power Potential

ISO -- ISO or isolated power is simply slugging percentage minus batting average. I like to look at ISO instead of SLG because it distills a prospect's power potential to a single number, uninfluenced by batting average. The minor league leaders last year (some guy named Chris Carter) had a mark over .300, but you'll rarely find someone posting such a high mark in the major leagues.

Doubles and Extra-base hits -- Power is often the last tool to fully develop, and doubles can be a leading indicator of future homerun power. Carlos Quentin, for instance, hit 55 homeruns in the minors, but nearly doubled that amount with 103 doubles almost exclusively in hitter-friendly parks. His 40 homer breakout last year was surprising, but not entirely unexpected given his minor league track record.

In lieu of doubles, I often like to look at extra-base hits and extra-base hit percentage, the number of extra-base hits divided by total hits. Brandon Allen had an XBH% of 42% in '07 and an amazing 51% last year. He has power potential that can match up with the game's elite prospects. An XBH% of 30% is good, but I like to use XBH% more as a tool to tell me what type of power a player has, and to gauge year-to-year progression, rather than as a measure of future major league viability.

As for Fields, he posted a XBH% of 35.8% in his first full season (AA), and upped it to 39% in his '06 minor league breakout which featured 36 non-HR XBH versus 19 homers, a sign of a more HR power in the future. His XBH% peaked at 41% in the two months before his call-up in '07.

Fluke Check

BABIP -- Batting Average on Balls In Play, simply a player's batting average when they don't strike out or hit a homerun, is a good check to see if a player's triple slash line is repeatable in the majors. I'm sure most of you are familiar with BABIP, which averages very close to .300 in the major leagues. It's not as reliable in minor leagues for a number of reasons, but it still has value as a fluke check.

For instance, in Fields breakout year, his first full-season in Charlotte, he hit .305/.379/.515. Breakout year, future high average, plus-power star, right?

Wrong. Fields had a completely unsustainable .395 BABIP that season. While your line-drive hitting top prospects, the guys who hit the ball hard much more consistently than your average minor leaguer, often have BABIPs well above .300, .395 is well outside the bounds of sustainability at the major league level.


You can easily remember these guidelines by distilling them into two questions: Does he make enough contact? and What happens when he makes contact?

What can we expect from Fields, given his minor league stats? First, I think we have to discount last season, when Fields pouted having not made the major league team, then struggled while experiencing a myriad of leg issues, one of which required surgery this past off-season. Statistically, for evaluation purposes, we should probably think of Fields as the player who got called up in June of '07 to replace an injured Joe Crede.

I wish Fields had a complete second season at AAA, then we could be sure that he really had made progress in his walk rate. Nevertheless, Fields figures to be a low-average slugger, who strikes out a ton, but hopefully will be able to post an average OBP thanks to a good batting eye. He's shown an ability to adjust and improve at every level, and he may be in the process of taking that next step at the major league level, shortening up his swing just enough -- it's still big -- to make more contact and raise that average.

Resources -- Has BB%, K%, ISO and BABIP
Baseball-Reference -- full minor league stats

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