I have a decent model for hitting prospects that is based solely on OPS (and age/league, of course). I'd like to take K% into account as well, but don't have a working set of equations yet. So my hitter predictions for this season will be based on the OPS version.
First, I took out all prospects who I can't rate: specifically, those with <150 PA at any one level, those in the DSL, and the older Cuban signees. The PA cutoff is a sample size issue, the DSL problem is that for players so young and with such high variance a statistical approach isn't especially warranted, and the Cubans break the system, which normally would reject such old players as already failed but doesn't account for their late arrival into US baseball.
Then, I took everyone and used my criteria (based on age, league, OPS, and K%) to determine who is a prospect vs an org. player. Organization players get an equal 0.5% chance of becoming successful MLB players, and are usually the guys you wouldn't bother ranking. If you don't see a name on my list, and the player had >150 PAs at a single level, I see him as a non-prospect.
Finally, I used my model to generate three values: chance to make the majors, chance of success (>1600 PA and >1 rWAR), and chance of stardom (>1600 PA and >14 rWAR). Note that the chance of success INCLUDES the chance of stardom (and the chance to make the majors includes the chance of stardom and success). I then ranked them based on these values.
These rankings are based ONLY on performance THIS PAST YEAR. Really, I should be taking previous years into account, but I don't have a solid mathematical way to do this just yet.
I'm not aware of any unusually promising hitters in the DSL and wouldn't really be able to evaluate them anyway. So the last step is to add the promising unrated hitters (Hart, Sisco, Murphy, Heim, Urrutia, and Alvarez, let me know if I missed anyone) into the list. I did this based on feel, reading scouting reports, and other rankings.
Sisco's upside is high enough that I'll put him ahead of Marin. Hart and Murphy performed similarly; I'd put them both ahead of Sawyer. Heim was poor, but I'll put him ahead of Rosa since he's only 18. Alvarez and Urrutia I have no idea what to do with, at all, so I put them about where most other rankers did.
Final Ranking, Age: OPS/K%. League Maj%/Succ%/Star%
1. Jonathan Schoop, 21. .697/19%. IL 100.0%/71.0%/48.6%*
2. Michael Ohlman, 22: .934/21.9%. CARL 62.3%/17.5%/ 6.6%
3. Henry Urrutia, 26.
4. Dariel Alvarez, 24.
5. Chance Sisco, 18. GCL
6. Adrian Marin, 19: .667. SAL 47.4%/16.0%/ 7.7%
7. Christian Walker, 22: .822/17.2%. CARL 45.5%/12.5%/ 4.3%
8. Trey Mancini, 21: .832/15.1%. NYPL 36.6%/ 9.4%/ 6.6%
9. Alex Murphy, 18. GCL
10. Josh Hart, 18. GCL
11. Wynston Sawyer, 21: .720/18.9%. SAL 26.2%/ 7.6%/ 4.1%
12. Glynn Davis, 21: .629/18.0%. CARL 39.1%/ 7.7%/ 1.5%
13. Conor Bierfeldt, 22: .862/24.8%. NYPL 23.8%/ 6.2%/ 0.9%
14. Mike Yastrzemski, 22: .781/18.7%. NYPL 14.7%/ 4.3%/ 0.9%
15. Lucas Herbst, 22: .696/17.1%. SAL 14.7%/ 2.0%/ 1.1%
16. Jonah Heim, 18. GCL
17. Garabez Rosa, 23: .665/16.1%. EL 30.8%/ 2.3%/ 0.0%
*There weren't enough 21-yos in the INTL to develop an OPS relationship, so the best predictor for Schoop is the mean results of those in his age category. He was below average in OPS among this group but not by much.
The biggest caveat of my system, I think, is that it doesn't account for a player's position/potential defensive value. Players like Marin, who play a premium position and have positive defensive scouting reports, are likely underrated; players like Walker or Bierfeldt, who are limited defensively, are likely overrated.
Also, please remember how this sort of prediction is supposed to work: if you put all of my 8-12% predictions in one pile, you should find that 1 in 10 of those players was ultimately successful.
Finally, the system is new and certainly in need of refinement and testing. I think I've made a good start to a stastical prediction system for prospects, but what I have now is along the lines of Marcel, using some simple basic numbers, rather than a tuned mathematical tool.
Also potentially of interest are the values for the guys who were traded:
*Nick Delmonico, 20: .819/22.4%. CARL 82.0%/46.2%/30.5% - would be #2
*Xavier Avery, 23: .798/23.7%. EL 57.8%/23.9%/ 8.4% - would be tough to rank, his age-23 performance in the Eastern League was actually very solid
*Ty Kelly, 24: .771/14.3%. EL 42.8%/ 9.3%/ 3.0% - would be after Hart, ahead of Sawyer
*LJ Hoes, 23: .808/13.0%. INTL 88.0%/35.5%/27.8% - would be #2, or #3 after Delmonico
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