IsoD, for those of you who are unfamiliar with it, is short for "Isolated Discipline" and is calculated by OBP-AVG. It basically tells you what percentage of plate appearances result in a walk for that particular player. It tends to go along with IsoP (Power), which is SLG-AVG.
In my limited evaluation of players, IsoD and IsoP are the true measures of a player's talent. There's always some baseline batting average, but you can dismiss most of the fluctuations in that to BABIP. Issue is, this only really works for more experienced players (more stats to look at).
Let me give you somewhat of a numerical example.
Derek Jeter had one of the worst seasons of his career in 2008, and if you look at strictly OPS, it WAS the worst of his career. Does this mean that he's washed up and done for? Well, let's take a look at the numbers.
As I mentioned earlier, BABIP (lucky drops, gaps, etc) can account for year-to-year fluctuation in a player's batting average. If that player's IsoD and IsoP are consistent with his career norms, then I wouldn't read too much into a drop in OPS.
In 2008, Jeter batted .300/.363/.408, good for an OPS of .771. Not really great (especially if you take his defense into account), but a .300/.363 player is hardly something to sneeze at. Argue salary on your own time.
Anyhow, Jeter is a career .316/.387/.458 batter. So his batting average is a bit lower than his career numbers. Let's look at his IsoD. For 2008, it's .363-.300, or .063. For his career, it's .387-.316, or .071. So comparing .071 to .063, we're really not talking about a massive difference, only .008. If you look at his career IsoD's (per season) and get the standard deviation on them, that shows up as .010. That just means that this IsoD fluctuation is within a year-to-year fluctuation.
The thing that somewhat interests me is the fact that his AVERAGE is down .016 AND his IsoD has dropped. It should also be noted that his career IsoP is .142, and in 2008, it was only .108. That's a more significant drop.
To perhaps explain the low Average, we can look at his BABIP (Batting Average on Balls in Play). In 2008, Jete's BABIP was .336. That's high, compared to league average, but when looking at an individual batter, you want to compare it to his career BABIP instead - which is .361 for the Captain.
What have I gleaned from this? Something is happening with Jeter outside of simple fluctuations, or at least something happened in 2008.
From this point, we've determined his power swing has diminished. Turn him over to the team Strength and Conditioning coach, get him some Wheaties, get him to work on putting the ball into the gap. He'll be fine.
But I never settle at this. Anecdotal evidence is nice, but can I tie numbers to this situation?
My last (easy to find) source is his Plate Discipline data, which is generously provided by Fangraphs.com.
In 2008, Derek Jeter swung at 23.7% of balls outside of the zone, which is only a 1.8% increase from the previous season. However, this number is up 8.2% from 2005, when Jeter posted a healthy .389 OBP (and a .080 IsoD along with it). To complement the increase in out-of-zone swinging, his contact rates outside of the zone ALSO went up 18.6% since 2005 (Out-of-zone contact can lead to poorly hit balls, resulting in doubleplays, etc - unless your name is Vlad Guerrero, then it's a HR). His in-zone discipline has remained right about the same from previous years.
So what have we learned just from looking at like 8 numbers on Fangraphs? Well, 1. We know he's gotten unlucky, based on the .025 drop in his BABIP. 2. We know he's getting a bit impatient, from both his low IsoD, and his increase in swinging at (and making contact with) balls outside of the strike zone.
So now when your Yankees fan friend comes up to you and says "hey you're a big [expletive] baseball stat dork, tell me why Jeter [expletive] my [expletive][expletive] last season" and you can say "Well, let's see. He isn't walking as much as his career average (IsoD!) and he isn't hitting the ball for as much power as normal (IsoP!), maybe he's just getting impatient (Swinging %!!!)"
Look at you, you sabermetrician, you!