The defensive spectrum is a sabermetric concept that attempts to quantify the differences in value among baseball players by their defensive position. The idea is as follows:

Some defensive positions are more difficult than others. Since defensive skill and hitting skill are presumed to be independent (or close to independent), the average level of offense provided by an adequate fielder at each position will vary. The defensive spectrum, as it is generally agreed upon, is as follows (from most difficult fielding position to least):

Catchers' position on this list is somewhat unclear. Part of this is that the sabermetric community is at odds with conventional baseball wisdom regarding the proper role of a catcher. If the sabermetricians' argument that the only defensive metric that is meaningful for catchers is how well they prevent stolen bases, then catcher would fit near the bottom of the spectrum, around the corner outfield positions, in line with how well the top half of the catchers in baseball hit, and presuming that it costs nothing to convert an adequate first base or outfield prospect to a below-average defensive catcher. If there is any merit, actual or psychological, in the argument that catchers' ability to handle pitchers is an important part of their defensive skill, then catchers should be around the same level as shortstops and second basemen, in line with the current average offensive ability of major league catchers.

An important part of sound player development is to find players who are at the left end of the spectrum when they are acquired and show promise as hitters. If they don't progress defensively but hit well, they can always move right to an easier position. Frequently (if you read scouting notebooks) you will see scouts and analysts observe that a player has "moved a long way right on the defensive spectrum". This is a warning sign when the player is still in the minor leagues but can be acceptable in the majors. For instance, Gary Sheffield came up to the majors as a shortstop, moved to third base, and is now a corner outfielder, but his hitting talent makes him a solidly above-average player even in left field.

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