After putting all the stats together for this series, I realized that (though they made sense for me to calculate) they weren't necessarily "user-friendly." The toughest thing was that -- for the stats based on the "necessary conversion rate" of "random-y" balls-in-play to "random-y" singles -- "low" was good for hitters and "high" was good for pitchers, which is completely counterintuitive to what we are used to.
That, plus the "Plate Skills" metric (indicator of OBP) was on a completely different scale from the "Production" metric (indicator of SLG).
So, what I have done is converted both those key stats to a scale that is intended to be similar to OPS+, which which most folks are familiar.
The key difference, however, is that for OPS+ the score of "100" is league average. For prospects, the "100" score is what I have found to be roughly the minimum score for guys with a shot at the majors.
Therefore, for Plate Skills ("Hitter's +/-"), instead of "0" = "100" (which would indicate minor-league average), I set "+1.5" = "100" since that is "roughly genuine prospect minimum."
For Production ("Plausibility Index"), the average was more in the .360 range (higher some years), but I set "100" at .330, which is, again, about as high as one can get and be a reasonable year for an MLB prospect.
The third number is not initially as intuitive, but it is the composite of the two other numbers. It is determined the same way that OPS+ can be determined from OBP+ and SLG+.
For example, Ichiro's career OBP+ (his OBP/leage avg. OBP) is 112 (.365/.325). His career SLG+ is 101 (.419/.414). Those two numbers contribute to his career OPS+ of 113. It is "+12" plus "+1" that gets you the "+13." The "100s" don't really matter ... it is the "plus" or the "minus" that matters.
Another example: Brendon Ryan ... OBP+ of 95 (.306/.323) and SLG+ of 80 (.327/.407). So that's "-5" and "-20" for a total of "-25", which produces an OPS+ of 75.
So you can always get the third number from the first two, but not by adding or averaging, but by combining the differences from 100 and then taking that combination as a difference from 100.
That gives us a series of three numbers: For MLB players ... OBP+, SLG+, OPS+. For prospects ... Plate Skills, Production, Composite.
How to interpret, using examples from MLB vets:
Excellent at everything: Prince Fielder ... 117-127-144
Not quite as excellent at everything: David Wright ... 114-120-134
Plate skills, less power: John Olerud ... 118-111-129
Fewer plate skills, more power: Giancarlo Stanton ... 106-134-140
Fewer plate skills, not as much power: Mark Reynolds ... 98-111-109
Here are the career scores from the 2012 Mariners veterans:
Miguel Olivo ... 83-100-83 (terrible plate skills, decent power, but not enough to make up for it)
Brendan Ryan ... 95-80-75 (tolerable plate skills, no power, better have a good glove)
Ichiro ... 112-101-113 (his career SLG is a point higher than Olivo's)
Chone Figgins ... 105-87-92 (career totals reflect a classic middle-infielder/leadoff profile, but it didn't happen in Seattle)
Franklin Gutierrez ... 95-94-89 (not bad for a gold-glove CF, really ... if he can stay on the field)
OK, I hope that gives you a sense of how the series of numbers works. If one of the numbers is below 100, then the other number ought to make up for it ... or you'd better have a really good glove.
And for prospects, the 100 is set at the minimal for a guy with a real shot, so 102-100-102 (Kevin Rivers, at age 23 in Low-A) only indicates that he's on the fringe, even if he weren't old for his level (and I'm a Rivers fan).