The defensive spectrum is a term coined by Bill James in the 80s, and attempts to show the relative scarcity of defenders able to play a certain position. In order of most common to least, it looks something like this:
Many people would describe this as a ranking from easiest to hardest, but that misses out on a lot of the subtleties of the game. Think about it: would you really expect a catcher to be able to play a competent centre field? No, obviously not. This is because there are many different skills a defender can have, and each position weights these skills differently. Matthew and I have come up with a quick list of skills a defender might have:
Reaction Time (RT): How fast the player reacts to a batted ball.
Reaction Direction (RD): Accuracy of initial break.
Routerunning (RR): Minimising the deviation a linear path from the initial fielding position to where the play must be made. Requires a good feel for the motion of the ball in flight and early compensation for spin/wind altering the trajectory of the baseball.
Speed (SP): Raw footspeed. Fairly self explanatory.
Hands (HD): The ability to make clean catches, field the ball off the ground, and transfer the ball quickly and consistently.
Arm Strength (AS): How hard the fielder can throw.
Arm Accuracy (AA): " accurately ".
There are other, more minor things to consider (feet, positioning, teamwork, etc), but by and large these seem to be the big seven. Now that we have our fielding toolbox, we can guess at how important each is to a given position:
These weights are not meant to be definitive; they are simply a reasonable guess at the skills required for each position. In order to establish the true weightings, we need some method of measuring each skill on a player as well as well as an overall defensive metric that people are happy to use (perhaps some average of PMR, UZR, RZR, etc). I know Matthew's interested in developing various ways of evaluating these tools, and Tangotiger's fan surveys are also heading along this route, albeit in a much more subjective fashion (which may work better, who knows?).
By framing the defensive spectrum in this manner, we can look at the potential merits of position changes far more clearly than by use of the unweighted spectrum alone. It's a cool little idea which I think definitely deserves further thought.
*This is not a strict definition: 2B and CF get flipped around a lot, and individual parks may also have an effect on the ordering.