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Building A Bullpen - Part III

Slight change of plans. I said that I was going to look at relievers signed by the Cardinals last week -- instead I'm going to stick with looking at some trends among relievers. I wasn't particularly thrilled with the results of the DIPS data as far as what it told us about individual relievers. Sure, we identified which teams bullpens had the best peripheral statistics but when we looked at the core relievers in their pen, it didn't seem particularly enlightening. So back to the drawing board. . .

I'm going to present some numbers today but they're simple and they're in picture form. Before we get to that, let me tell you where these numbers are coming from since I imposed some artificial limitations to my dataset. I took all the pitchers seasons from 2000-2006 and removed any pitcher's season where they started a game since we only want relievers. I know that eliminates players like Brad Thompson who probably had as many relief innings as he did starting innings in a few years but players like that just muddy up the picture. Then I eliminated any season that accounted for less than 20 innings. I'm not particularly interested in relievers that get hurt early in the season or marginal relievers that shuffle up from the minors like Andy Cavazos. The latter don't tend to stick in the majors long and if they do manage to rack up a 20 inning season that will still be in the dataset. I'm defining a players age as whatever age they were before September 1st. So if a player turned 27 in September 2006 they're going to be in my 26 age group for that season. If they turned 27 in August, they'll be in the 27 age group for that season.

Let's look at the distribution of relievers by age.

We've got something of a bell curve here with 29.9 as the average age for all relievers. While 30 isn't the most probable age group, it's the center of our distribution. Not a particularly surprising result. We can also see that ages 26-31 make up about 50% of our total number of relievers. I'm more interested in the tail of this data though. Around age 36 we see a pretty steep drop in the number of pitchers that qualify. There's only 66% as many relievers that record age 37 seasons as age 36 seasons. Moving forward there's only a third that stick around from 36 to 39. That looks like a dangerous age to live in for relievers. Once you hit about 41 some old timers that take joy in mucking up my nice distribution but for the most part they don't fare well (Jesse Orosco, I'm looking at you). We're going to ignore them going forward and focus on ages 21 through 40.

Let's see what the average number of innings pitched by our relievers is.

Now remember that I've created a artificial offset of 20 for this graph or these numbers would all come down. It looks like those 21 and 22 year old relievers get used pretty heavily upon their arrival -- a few names in this group include Francisco Rodriguez, Chad Cordero and Joel Zumaya. The 23-27 age group is probably where pitchers are coming up around mid-season to fill in for an injury which holds their numbers down. The unheralded middle relievers and journeymen emerging from the minors. You can see from ages 28 to 37 the graph is relatively constant ranging between 55 and 60 IP on average. After that it's a precipitous decline in workload.

We can see that those young guns that get called up are pitchers that feature high strikeout rates -- again, think K-Rod and Zumaya or Jonathan Broxton. Those are your dominant young relievers. Once you get past the truly spectacular players the more uninspiring middle relievers filter up and dilute those K rates. From age 25 on, it's a slow decline in missing bats. For those few relievers that we saw surviving age 36, the decline is a little more dramatic.

This explains why those older relievers are able to stick around. They get real stingy with the free passes. I compiled a K/BB graph but it was rather uninteresting as it sat around 2K/BB for all age groups. Again we see that those relievers coming up young feature better BB rates than what we'd expect looking at a linear trend line. Once we hit 37, the BBs begin to climb again.

I'm much happier with what this data showed us. Young relievers survive on a combination of good stuff with a lack of control. As they age, the stuff deteriorates but the control improves; that's really they're only option or they'd find themselves out of baseball in a hurry. It looks like relievers around 36-37 are entering a real danger zone as far as their peripherals are concerned. Unless a pitcher has been above these averages consistently into their old age, I'd want to avoid them. Russ Springer version 2007 immediately comes to mind as someone whose age 38 season suddenly bucks the trend of their previous seasons and what we see in these graphs. Setting aside the extenuating circumstances of that signing, Springer could rapidly turn into a pumpkin this coming year.