I'm going to take a step back from baseball for a second to do some meta-scientific thinking. This is important because so much of what I talk about when I talk about baseball is essentially scientific analysis. Therefore, it is important to understand what science is and what it provides us.
Webster's defines science as "a department of systematized knowledge as an object of study." That works, but I want to offer a much more concise definition.
Science: logical conclusions drawn from empirical observations.
This definition establishes the two key parts of science: the use of tests and observations to gather data and the analysis of that data to reach reasonable conclusions about what was tested.
It's important to understand the limitations of this approach. Science always entails assumptions, margins for error, limits in precision, and other dirty details associated with measuring something as complex as the real world. Science has to deal with all sorts of interferences from unexpected agents in the systems being tested.
This is not to say that the system is not powerful. Science is an absurdly powerful methodology when properly applied. We have accumulated massive amounts of useful knowledge through science.
But what does science actually leave us with? After all the tests and all the observations and all the analysis, science presents us with a model of reality. These models work given certain assumptions. Newtonian physics holds up quite well for many real world physics problems, but breaks down on the molecular and atomic levels. These models aren't reality, but they provide us with a necessary characteristic for any sort of practical application: simplicity.
The models that science provides us are always grossly simplified compared to what actually exists. Newtonian physics treats complex combinations of many different types of atoms as solid bodies. Objects don't have a "mass;" mass is a measurement designed to quantify something otherwise impossible for us to grasp. Without this simplicity it would be impossible to apply scientific findings in any meaningful way.
So what does all this have to do with baseball?
Science applied to baseball provides us with simplified models and measurements that allow us to analyze baseball in ways we never could before. In fact, in one way, baseball is so much better a subject for science than the physical world: baseball has been categorizing and measuring itself in detail for a hundred years. It's one of most massive controlled experiments in history.
When I talk about Derek Jeter's VORP, it's the same as if I was talking about his mass. Derek Jeter doesn't "have" a VORP. He's a complex individual in a complex system. VORP is one way that we try to quantify his effect on that complex system. Is it a massive simplification? Absolutely. It would be useless if it wasn't.
Science in baseball is often skewered for being a geeky simplification of a grand human endeavor. Of course it is! That doesn't limit its analytical power. We can still reach powerful conclusions from a simple model of baseball.
And with that, I'm going to begin talking about the model that we use for a baseball player, the observations that formed it, and how we can use it to draw conclusions about real baseball.