[RchW]

I think it is as simple as total number of pitches. A pitcher throwing strikes effectively will by definition throw less pitches and seem to last deeper into games than a nibbler who goes 3-1 and 3-2 on a lot of batters.The big knock on modern pitching is that they don't throw strikes.

Later in games some pitchers maintain effectiveness longer than others. I've always had the impression that Pedro Martinez was a 6 or 7 inning pitcher with a big drop off after 90-100 pitches. I looked up some stats on Baseball reference.com and found that just about every decent starter last year was around 7 innings per start.

Thirty years ago guys like Carlton and Jenkins or Palmer rarely missed the strikezone so the batters went up swinging and they would go 9 innings on 90-110 pitches.

If you extend the same idea to relief pitchers say 100 pitches -25%(arbitrary value) for pitching several days in a row, they have 75 pitches to give. So a set up guy may appear in 5 of 6 games in a week 5X15 = 75 pitches. A closer may pitch a little less often say 4 of 6 games but may throw more pitches per appearance say 20 and you get 4X20 = 80.

Again I'm just picking these numbers out of the air but you could suggest a table something like

First 60 pitches 100%
Next 20 90%
Next 20 85%
Next 10 75%
Next 10 60%
Next 10 50%

Relievers may drop off at 40 pitches but the concept would work for relievers who are used a lot with some formula to restore pitches for days off.