Hank Aaron, Barry Bonds, Aging and Park Effects

This issue came up recently on the SABR list. Is there something unusual about Bonds' performance as he aged? One poster said that both he and Aaron's HR% relative to the league average showed a similar pattern as they aged.  But I think that did not tell the whole story.

So here is what I posted.

Using Baseball Reference, below are the neutralized SLGs by age for Aaron and Bonds. That is adjusted for league average and park effects (and I recall that in AT&T park lefties did not hit very many HRs and that a huge portion were by Bonds himself-more on this below).

Aaron does a bit better in his late thirties than earlier. But his .677 at age 37 is only .025 better than his best year at age 25.

Bonds best year was .854 at age 36. His previous best year was at age 28 with .670. That is .184 higher. His 2nd, 3rd and 4th best years were at ages 37 (.800), 39 (.777), and 38 (.727). So even that .727 was .050 better than anything before age 36.

Aaron's neutralized career SLG thru age 35 was .576. From ages 36-39 it was .591 (for a gain of .015). So, yes, a little unusual to see a player do better at such and advanced baseball age. But nothing compared to the improvement Bonds had.

Bonds' neutralized career SLG thru age 35 was .566. From ages 36-39 it .793. That is a gain of .227. That far surpasses Aaron's gain.

Here are Aaron's stats

20    0.445
21    0.533
22    0.571
23    0.608
24    0.553
25    0.652
26    0.587
27    0.600
28    0.617
29    0.618
30    0.526
31    0.572
32    0.540
33    0.602
34    0.547
35    0.617
36    0.549
37    0.677
38    0.510
39    0.631
40    0.490
41    0.361
42    0.396

Here are Bonds' stats

21    0.416
22    0.479
23    0.515
24    0.447
25    0.577
26    0.526
27    0.647
28    0.670
29    0.637
30    0.567
31    0.604
32    0.567
33    0.599
34    0.591
35    0.659
36    0.854
37    0.800
38    0.727
39    0.777
40    0.667
41    0.514
42    0.535

Using Baseball Reference, here is what I came up with for Bonds' HR% (HR/AB) over 2001-4. Home 13.3% and road 12.2%.

For Aaron over 1970-73. Home 10.5%  and road 6.7%.

Using Bill James Handbooks, here are the HR park factors for left-handed batters in San Francisco

2001-57
2002-55
2003-75
2004-92

Under a 100 means below average (2003-05 was 82). So a number like 57 means the HR% for lefties in SF was 43% below league average.

Using another Bill James book (I think it was called something like the All-Time Source book), here are the HR park factors but for all batters in Atlanta, not just L or R (although my recollection is that Atlanta was symmetrical)

1970-157
1971-152
1972-139
1973-143

157 means that the rate was 57% higher than the league average (not totally sure if those older park factors were for rates or just total HRs).

But in any case, it looks like Aaron got alot more help from his park.