I think that the decline in DH performance relative to the league and the decline of full-time DHs are related, because they both stem from the construction of pitching staffs in general, and the modern bullpen in particular. In 1973, the first year of the DH, teams commonly carried 10-11 pitchers on their 25-man rosters. Now they usually have 12, sometimes as many as 13. That leaves less room for a full-time player who can’t play in the field and more need for positional flexibility.A player who can, for example, fill in at second base, third base, and all three outfield position is probably not going to be the slugger that a bat-only full-time DH like David Ortiz is.
In the article I get more stats-heavy than I tend to be here. Here are some of the terms I use:
- wRC+ is a measure of offensive performance. A wRC+ of 100 is league average. A score of 105 is 5% above average, 95 is 5% below average.
- OPS+, which I've occasionally used here, is, like wRC+, scaled so that 100 is league average. It's on base percentage plus slugging percentage, adjusted for the effects of ballparks.
- wOBA is another measure of offensive performance. It's scaled like a batting average; a wOBA of .300 is pretty good.
- The defensive spectrum ranks defensive positions by difficulty. The toughest position is shortstop, followed by second base, center field, third base, right field, left field, first base, and designated hitter. Catchers and pitchers require special skills that involve more than just catching and throwing, so aren't on the spectrum, but catchers are generally thought to be on the hard end of the scale.