Does plate discipline come with age?

A week ago I did a piece on quantifying plate discipline with the metric DISC+. The results, that of which you can read here, yielded a year to year R^2 of .77, telling us that we captured a real repeatable skill in DISC+.

Now that we have a simple way of calculating a metric that at least can map the very essence of a player's plate discipline skill -- we can look into the correlation between age and patience at the plate.

It has long been considered a given that plate discipline is a skill that becomes more developed with age. With a way to quantify discipline in our hands, we can at least try to distinguish the correlation -- whether it exists or not -- between age and plate discipline.

Before we do that, I should announce that a few changes have been made to the overall DISC+:

1) Z-CONTACT/Z-SWING

2) (1-ZONE)*O-SWING*(1-O-CONTACT%)

3) PITCHES/PA

Yielding the formula:

((Z-CONTACT%/Z-SWING%)/(Z-CONTACT%(lg)/Z-SWING%(lg))*100+*((1-ZONE(lg))*O-SWING(lg)*(1-O-CONTACT%(lg))/((1-ZONE)*O-SWING*(1-O-CONTACT%))*100+(PITCHES/PA)/(PITCHES(lg))/PA(lg))*100)/3

Using these principles, we will adjust them for league averages to be a plus stat -- like we did in the previous article. Doing this, the year to year correlation between DISC+'s jumps form 77% to 84% -- a really fantastic improvement.

Moving on, lets look at a chart for the average DISC+ totals for each age, 21-40, since 2002:

Results

Age	DISC+
21	120.43
22	99.71
23	101.82
24	103.96
25	107.79
26	107.35
27	108.45
28	107.38
29	109.33
30	106.21
31	107.69
32	110.15
33	112.85
34	110.42
35	110.17
36	109.13
37	106.93
38	124.62
39	128.58
40	120.86

You can see that there seems to be little relationship between age and DISC+; but interestingly enough, the 38, 39, 40 age seasons was where we saw an uptick in the average discipline totals.

However, there is no significant correlation to age with and R^2 of .01 -- but DISC+ is a significant predictor with a p-score of 0.0006947.

Even in this small sample size we see an interesting trend -- 21 year olds exhibited above league average plate discipline. However, in the early 20's, the average is around below league average; meanwhile, in ages 25-31, players exhibited a DISC+ around the league average of 108.

Interestingly enough, a trend upwards occurs in plate discipline during the 38, 39, and 40 year seasons that makes you wonder if age is a function of discipline. This, however, could be a function of the lack of qualified seasons from 38, 39, and 40 year olds since 2002 -- so this result also screams small sample size.

In the end, the numbers tell us that there is no significant trend behind age and discipline as quantified by DISC+, even as much as I want to believe in its existence.

For the sake of discovery, let's take a look at the player seasons with an age over 38 -- the time in which we saw the most significant trend upwards in discipline -- and look at the biggest gainers and losers during that age period:

Name	Season	Age	Gain
Omar Vizquel	2005	38	17.62
Rafael Palmeiro	2003	38	8.69
Craig Biggio	2005	39	8.07
Steve Finley	2003	38	5.76
Raul Ibanez	2010	38	4.77
Ichiro Suzuki	2012	38	4.01
Barry Bonds	2003	38	3.86
Luis Gonzalez	2006	38	3.69
Kenny Lofton	2007	40	2.70
Craig Biggio	2007	41	1.25
Jamey Carroll	2012	38	0.98
Omar Vizquel	2006	39	0.85
Frank Thomas	2007	39	0.74
Craig Biggio	2006	40	0.54
Omar Vizquel	2007	40	0.35
Luis Gonzalez	2007	39	-0.64
Derek Jeter	2012	38	-1.40
Raul Ibanez	2011	39	-6.14
Craig Biggio	2004	38	-15.73
Edgar Martinez	2004	41	-20.70
Jeff Conine	2004	38	-23.40
Steve Finley	2004	39	-24.27
Rafael Palmeiro	2004	39	-30.70
Barry Bonds	2004	39	-32.95

Palmeiro and Bonds made jumps in their DISC+ totals in the 2003 season -- both at the age of 38. Subsequently, the duo both took ugly dips downwards in 2004 at -30 dives in their discipline rating.

Also, as you can see, the amount of lost discipline far exceeds that of which was gained by the population; only complicating our findings on the overall uptick in the age 38 - 40 seasons.

In the end, there may be no significant correlation here, but given the fantastic year to year consistency of DISC+ a player who has a plate discipline skill is unlikely to lose it when he ages. For guys like Omar Vizquel and Marco Scutaro, their fantastic plate discipline had less to do with their age and experience and more with there career skill. With the mean of DISC+ around 108 and the standard deviation of season to season gain being only 9, it is not unreasonable to think of DISC+ as a skill rather than a function of experience. More often than not these skills do not deteriorate. If a player enters the the league with a good eye, he will retire with a good eye. If a player enters the league with a terrible approach, his DISC+ will stay fairly constant with the original quantified approach.

Finally, lets take a look at two players who entered the league with completely different skill-sets to back up the above points -- Vladimir Guerrero and Jason Kendall:

Guerrero starts his career off with below average plate discipline skills, while Kendall begins with exceptional plate approach. Despite a late surge towards the end of Vladimir's career, he ends up right where he left off -- below league average. Despite Kendall's prime years discipline being better than where he started off as a rookie, he regressed back to where he started his career. Although this is only a snapshot into two player profiles, you can see discipline has more to do with skill and entry rather experience and tenure.

What do you guys think? Do you believe that age is a component of discipline and that the secret is hidden within?

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