How big was Bryce Harper's decline after his great season in 2015?

In 2015, at age 22, Harper had a 198 OPS+. The only player to have a higher OPS+ at age 22 or younger was Ted Williams with 235 in 1941 (the year he batted .406). And Ty Cobb is the only other player in this age group to reach 190 with 193 at age 22.

To look at how great young hitters did in the years immediately after their high OPS+ season, I had the Baseball Reference Play Index call up all the players age 23 or younger who had at least a 170 OPS+  with a minimum of  400 PAs.

The table below shows what they did at either age 22 or 23 and then the next three seasons. Avg is actually their OPS+ over the next three years combined, not a simple average.

Player	OPS+	Age	Yr.1	Yr.2	Yr.3	Avg
Albert Pujols	187	23	173	168	178	173
Arky Vaughan	190	23	148	133	141	141
Boog Powell	176	22	112	158	104	126
Bryce Harper	198	22	114	156	133	133
Eddie Collins	170	22	150	162	158	157
Eddie Mathews	171	21	172	170	143	161
Eddie Mathews	172	22	170	143	154	156
Eddie Mathews	170	23	143	154	120	139
Frank Thomas	180	23	174	177	212	186
Jimmie Foxx	173	21	161	140	207	171
Joe Jackson	193	23	191	192	156	181
Jose Canseco	170	23	147	159	157	156
Ken Griffey Jr.	171	23	171	122	154	153
Mel Ott	174	23	138	168	157	155
Mickey Mantle	180	23	210	221	188	206
Mike Trout	179	21	168	176	172	172
Mike Trout	176	23	172	186	199	185
Reggie Jackson	189	23	127	144	149	141
Stan Musial	177	22	174	183	134	163
Stan Musial	174	23	183	134	200	173
Ted Williams	235	22	216	215	205	212
Ted Williams	216	23	215	205	189	203
Tris Speaker	170	22	157	189	182	177
Ty Cobb	193	22	205	196	200	201
Ty Cobb	206	23	196	200	194	197
Willie Mays	175	23	174	146	173	165

There are 26 cases but some player appear more than once (Mathews, Trout, Musial, Williams, Cobb). So if we only look at the number of players instead, we have 20 cases. 13 of those 20 players had at least one season of a 170 OPS+ or higher in the next three years (starting from the first season they did it-Mathews did it first at age 21 and did it again at age 22 & 23 but not at 24, 25 or 26).

So most young players who have a great season have another one pretty soon after. Not Harper. His best season of the next three is 156.

The next table shows the difference between each player's great young season and the average of the next three, sorted by largest gain down to biggest decline.

Player	OPS+	Age	Avg of next 3 seasons	Diff
Mickey Mantle	180	23	206	26
Mike Trout	176	23	185	9
Ty Cobb	193	22	201	8
Tris Speaker	170	22	177	7
Frank Thomas	180	23	186	6
Stan Musial	174	23	173	-1
Jimmie Foxx	173	21	171	-2
Mike Trout	179	21	172	-7
Ty Cobb	206	23	197	-9
Eddie Mathews	171	21	161	-10
Willie Mays	175	23	165	-10
Joe Jackson	193	23	181	-12
Eddie Collins	170	22	157	-13
Ted Williams	216	23	203	-13
Albert Pujols	187	23	173	-14
Jose Canseco	170	23	156	-14
Stan Musial	177	22	163	-14
Eddie Mathews	172	22	156	-16
Ken Griffey Jr.	171	23	153	-18
Mel Ott	174	23	155	-19
Ted Williams	235	22	212	-23
Eddie Mathews	170	23	139	-31
Reggie Jackson	189	23	141	-48
Arky Vaughan	190	23	141	-49
Boog Powell	176	22	126	-50
Bryce Harper	198	22	133	-65

So Harper has the biggest drop off and he seems well ahead of the next biggest decliner, Powell. Considering how well he did at age 22 and where that ranks historically for young hitters, his performance the last three years may look disappointing.

I also looked at all players who had 1200+ PAs from ages 20-22 and an OPS+ of at least 140. Then I found their OPS+ over the ages 23-25 (not exactly, for example, Ted Williams missed three years in WWII so for him I just used his next three seasons).

Harper had an OPS+ of 154 over ages 20-22 and 133 over ages 23-25. That drop off of 21 is the 2nd biggest among this group of 22 players. In fact, 16 of the 22 actually did better over the next three seasons and the last time anyone dropped off as much as Harper was in the 19th century (Beckley). Pujols, Griffey and Trout are all fairly recent players who increased their OPS+ going from 20-22 to 23-25. Yet Harper has this big decline.

Player	OPS+ 20-22	OPS+ 23-25	Diff
Mickey Mantle	155	203	48
Ted Williams	182	212	30
Ty Cobb	176	201	25
Tris Speaker	152	177	25
Rogers Hornsby	153	177	24
Jimmie Foxx	162	184	22
Albert Pujols	154	176	22
Denny Lyons	147	164	17
Joe DiMaggio	147	163	16
Ken Griffey Jr.	146	161	15
Giancarlo Stanton	140	151	11
Mike Tiernan	146	154	8
Mike Trout	172	178	6
Sherry Magee	145	150	5
Mel Ott	156	160	4
Eddie Mathews	153	156	3
Fred Carroll	142	141	-1
Stan Musial	166	163	-3
Sam Crawford	145	141	-4
Al Kaline	140	129	-11
Bryce Harper	154	133	-21
Jake Beckley	142	119	-23

Where does Harper's 133 OPS+ rank all-time among 23-25 year olds who played at least half their games in the outfield with 1200+ PAs? 81st out of 99.

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