In an effort to drain every possible drop from the well, I have one final chapter in history of baseball rookies:

Effects on Winning:

First, I found that the percentage of rookies on a team correlated poorly to team success (.335 correlation coefficient). Then I took a look at teams' weighted ages and weighted experience level (i.e., weighted by percentage of games played) to see if they correlated to winning. They didn't either (0.3490 for team experience and 0.2157 for age).

Someone at Baseball Think Factory(which I still call Baseball Primer) suggested that perhaps it was a curvilinear relationship. That is, that teams with a large percentage of rookies and teams with a large percentage of vets would both perform poorly, while teams with a more balanced composition would perform best.

So I took my raw data and plotted Rookie Percent, Average Weighted Team Age, Average Weighted Team Experience, and Average Weight Team Debut Age (i.e., Age minus Experience). Unfortunately, they did not reveal a facile relationship to winning. I am unable to reproduce them here, but I'll hit the more salient points.

In all of the graphs (except debut age), teams that performed very poorly (with a winning percentage below around .375) had a large percentage of rookies and young, inexperienced players. As the winning percentage got lower the relationship got much stronger.

However, the rest of the graph ever so slightly did express this curvilinear relationship. But it was more like a dish than a bowl. Also, the percentage of rookies held between steady 15% and 20% for the rest of the graph. It was the most flat of all the graphs.

The Experience expressed the most arc, and the curve on the winning side of the graph (above .500) was slightly higher. With a winning percentage of around .400, the average experience level was between four and four and one-half years. It then increased to under six years at around a .475 winning percentage. The average seems to exceed six years slightly at a .550 to .575. Then it slopes down to about five and one-half years as the winning percentage approaches .700.

The Age graph, though flatter, expresses the slightly curvilinear arc that Experience did. At a .400 winning percentage, the average age approaches 28. It then holds steady between 28 and 29 until a .625 winning percentage. Again the above-.500 teams are slightly older (maybe by a half year). Above .625, the age drops quickly to between 27 and 28 (this may be due to the smaller sample size being heavily influenced by younger, nineteenth-century teams).

Finally, the debut age is the flattest of the lot, holding steady around 23 throughout. There is a slight age bump (to just under 24) with winning percentages under .375. Above .625, the graph dips close to 22. However, in both cases as the percentage gets more extremely, the graph returns to the norm (23).

So if there is any relationship, it's not very strong. If you're building a team for this year, just get the best players available regardless of age and experience.

Rookie Performance:

Finally, here's a rundown of how well rookies perform by decade, split up into batting and pitching. In each case, the typical statistics for all rookies are derived and then they are normalized by the major-league average (a la Baseball Reference's OPS+, though with no compensation for ballpark).

First let's look at some general numbers for all players:

You'll not that players are getting older and more experienced, and that their "debut age" is getting higher even as analysts claim that players are getting rushed to the majors. Also, the Forties stand out as the only decade with a debut age over 24, clearly being greatly affected by the dearth of talent during World War II. The Sixties, the first decade of expansion and of the amateur draft, saw debut ages drop dramatically. As the leagues stabilized in the Eighties, the experience went up, but it took more hits in the last two decades probably due to two more rounds of expansion (and potentially more foreign-born players in the majors).

Now let's look at the batters. First, here are some basic demographic numbers (Age+ is the average rookie age normalized for all players):

You'll notice that rookies have been getting older for the past five decades even though the disparity between rookie and veteran players holds steady (Age+ of 85-86). Also, teams have consisted of fewer rookies and they have gotten a smaller percentage of the at-bats since the Sixties.

Now here are the average and normalized rookie batting statistics:

And:

Well, gee, rookies really stink. Aside from stolen base percentage, they just are not very good at all. Let's see if the pitchers fare better…

Here are the demographic data for rookie pitchers:

The trends that we saw with the batters are evident here, but teams do seem slightly more willing to devote innings to rookie pitchers, at least more so than they commit at-bats to rookie batters.

Here are the rookie pitching stats (Note sub-100 normalized data represent worse than average values throughout):

That's not much more encouraging than the rookie batting stats. Again aside from one stat (Strikeouts per nine innings), the rookie pitchers are considerably worse than average.

It's no wonder teams have been eschewing rookies for veterans over the past five decades, right? That's why GMs are more willing to employ Wil Cordero and Jose Offerman rather than promote from within.

Well, that's the simplistic conclusion. Yes, rookies are on average worse than veterans. However, that doesn't mean every rookie will destroy your staff or become a sinkhole in your lineup. It also doesn't mean that every retread will salvage your team. It just means that GMs have to be a bit more discerning when it comes to the rookies they promote. Many will excel, but the GM will have to do his homework. It just seems that GMs historically (at least over the past five decades) have been more willing to take the easier route and just rely on a retread with a lengthy resume so that he'll have something to point to when they eventually fail.