This may be a very lame way of explaining what I'm getting at, or not. But I do realize now I should have found a picture of an open window.
Anyway ... imagine if you will ...
There is a little cannon, like the ones they use for practicing tennis or shooting hot dogs into the bleachers, and it shoots beanbags at the side of a house with a window. (We wouldn't want to damage the house, so we use beanbags.) The window is open, but the size of the opening is fixed. The cannon has bad aim, and moves up and down and back and forth in a totally random manner. For purposes of our thought experiment, the aim and randomness of the cannon cannot be changed, either. The beanbags are fed into the cannon from a hopper.
But ... some percentage of the beanbags make it through the window and into the house.
Every minor league hitter will have beanbags fired at the window on his behalf, and every minor league hitter will have a "Goal" of obtaining a given number of beanbags inside the house.
At the outset, with each minor league hitter getting an identical hopper-full of beanbags (that is, plate appearances), it is equally plausible for each hitter to reach the Goal. Some will and some won't, but that will be determined by the randomness of the cannon shots. At the outset, no one hitter is more plausible than any other.
But then we change the ground rules in two steps.
First, each hitter takes a certain number of beanbags out of his hopper, and places them directly in the house. These are walks, home runs and balls hit with authority (defined for measurement purposes as doubles + triples). In other words, a number of beanbags equal to XBH + BB comes out of the hopper and into the house. No cannon shot needed. Each hitter will be that much closer to the Goal, in an amount equal to the number of XBH + BB.
Second, each hitter takes a certain number of beanbags out of his hopper, and throws them in the dumpster. These are strikeouts. These beanbags never get into the house, and can't be used to reach the Goal.
After these two events, the plausibility of each hitter reaching the Goal will be different. Some will be very close to the Goal, and some won't. Some will have reduced their chances by removing a load of beanbags from their hopper. Hitters with similar-looking stats, might have very different plausibility because of very different strikeout rates.
After the two events, we've narrowed down our cannon shots to two of the six "plate outcomes." Walks, home runs and balls hit with authority go right in the house; strikeouts go right in the dumpster. The only "beanbags" left in the "hopper" represent "random-y" singles and "random-y" ball-in-play outs.
And ... we can calculate the conversion rate that each hitter needs to reach the Goal. That is, once we know how many beanbags are in the house, how many are in the dumpster, and how many are left in the hopper, we can calculate the percentage of beanbags that need to make it through the window in order to reach the Goal.
That is what the stat Plausibility Index is. The "Goal" is defined as an OPS of .890, which is the OPS that you get when you combine the threshold numbers that almost all successful major league hitters showed in the minors.
Then:
- given this hitter's actual rate of walks, home runs and extra-base hits [beanbags placed directly in the house], and
- given this hitter's actual strikeout rate [beanbags placed in the dumpster] ...
- what is the conversion rate of "random-y" singles [beanbags making it through the window] per "non-authoritative balls in play" [cannon shots out of the hopper] ...
- necessary to produce an OPS of .890 [the Goal]?
A very low Plausibility Index indicates that the hitter needs very few "random-y singles" to reach .890. The lowest for the Mariner system in 2012 was Mike Zunino's .112. For guys who played the full season, it was Leon Landry at .242. Once you get much over .350, then the likelihood of a hitter's actual success becomes less plausible (e.g., Jabari Blash at .373 and Ramon Morla at .355, despite decent-looking "regular" stats).
And, once we've set down that baseline, then we can consider that maybe some guys can "improve the aim of the cannon" or "expand the size of the window" (e.g., through speed). But, when we do, we need to recognize that it is something other than producing by hitting the ball with authority, drawing walks and avoiding strikeouts.