As of May 23rd, Brandon Morrow of the Toronto Blue Jays was leading the major leagues in strikeouts per 9 innings, with 11.8. Tim Lincecum of the San Francisco Giants was leading the majors in the number of strikeouts, with 75. These measures have long been used as a measure of the ability of a pitcher to strike out batters, with strikeouts per 9 innings being especially useful. One of the misleading parts of these measures, though, is that they do not reflect the strikeout frequency of the batters faced. If a pitcher faces batters that strikeout often, that pitcher would be expected to have a higher number of strikeouts and a higher rate of strikeouts per 9 innings. What would be helpful in determining the quality of a strikeout is an adjusted strikeout measure that reflects the strikeout frequency of the batter faced.
Actual vs. Expected Strikeout Ratio
To do this, you would need to look at the pitchers actual strikeouts in relation to the number of strikeouts the pitcher would be expected to have given the batters they face. For each batter, I calculated the strikeout frequency for the season, and took this as the frequency with which the average pitcher would strikeout this batter. I then compared the number of times a pitcher actually struck out the batter to the number of times it is expected that an average pitcher would strike out this batter. If the number of actual strikeouts is higher than the number of expected strikeouts, then the pitcher is striking out batters at a higher rate than average. If it is lower, the pitcher is striking out fewer batters than average. I have labeled this ratio the AESR (Average vs. Expected Strikeout Ratio), and it is simply calculated as the actual number of strikeouts divided by the expected number of strikeouts.
For example, in 2010, Mark Reynolds has struck out in nearly 40% of his plate appearances. If a pitcher faces Reynolds 10 times, given his propensity to strikeout, the pitcher would be expected to strike him out four times. If the pitcher struck out Reynolds six times, he would be doing better than average (AESR = 6/4 = 1.5). If he struck Reynolds out twice, he would be doing worse than average. Conversely, consider David Eckstein, who has struck out only four times in 158 plate appearances. Striking out Eckstein even once in ten tries would be four times better than average.
2010 Season
For 2010, as of May 23rd, the top 10 pitchers in strikeouts per 9 innings are (for pitchers with at least 30 innings pitched):
1 - Brandon Morrow (TOR) - 11.80
2 - Bud Norris (HOU) - 11.22
3 - Tim Lincecum (SFG) - 11.07
4 - Hisanori Takahas (NYM) - 10.69
5 - Carlos Zambrano (CHC) - 10.27
6 - Tommy Hanson (ATL) - 10.10
7 - Yovani Gallardo (MIL) - 10.07
8 - Clayton Kershaw (LAD) - 10.02
9 - Jered Weaver (LAA) - 9.92
10 - James Shields (TBR) - 9.68
For pitchers in the top 10 in strikeouts per 9 innings in 2010, most of them remain relatively high in the AESR rank. In this case, Morrow, the top pitcher in strikeouts per 9 innings, has ann AESR of 1.60, which means hestrikes out about 60% more batters than the average pitcher. Lincecum, who ranks at the top in AESR, strikes out 65% more batters than the average pitcher. Lincecum performs well at striking out both batters that strike out a lot as well as those that do not strike out as often. Lincecum has faced 83 different batters so far this year, of those 83 he has struck out two-thirds of them at least once.
One of the most interesting names on this list is Carlos Zambrano. Although he is fifth on the list of strikeouts per 9 innings, he ranks 28th in AESR (1.20), only striking out 20% more batters than average. Before he was placed in the bullpen, Zambrano was performing impressively in the strikeout category based on strikeouts per 9 innings, and his AESR of 1.20 is still better than average. However, relative to the other pitchers in the top 10, he has not performed as well in striking out more batters than expected.
The top 10 pitchers in AESR for 2010 are:
1 - Tim Lincecum (SFG) - 1.646
2 - Hisanori Takahas (NYM) - 1.606
3 - Brandon Morrow (TOR) - 1.604
4 - Jon Lester (BOS) - 1.553
5 - Clayton Kershaw (LAD) - 1.441
6 - Phil Hughes (NYY) - 1.404
7 - Justin Masterson (CLE) - 1.391
8 - Jered Weaver (LAA) - 1.390
9 - Yovani Gallardo (MIL) - 1.387
10 - Ricky Romero (TOR) - 1.381
On this list, the biggest jump from rank in strikeouts per 9 innings to actual vs. expected rank in by Phil Hughes, who rose 18 spots. One of the main reasons is that although Hughes does not strike out as many batters as others, of the 65 different batters he has faced, he has struck out 60% of them at least once.
While the comparison of actual to expected strikeouts does track with strikeouts per nine innings for many pitchers, there are some exceptions with which it does not track.
2009 Season
For the 2009 season, the top 10 list of pitchers with the best actual to expected strikeout ratio is shown below.
1 - Justin Verlander - 1.522
2 - Rich Harden - 1.512
3 - Tim Lincecum - 1.506
4 - Jon Lester - 1.486
5 - Zack Greinke - 1.468
6 - Javier Vazquez - 1.415
7 - Jake Peavy - 1.376
8 - Yovani Gallardo - 1.368
9 - Clayton Kershaw - 1.349
10 - Ricky Nolasco - 1.347
All ten of these pitchers were in the top ten in strikeouts per 9 innings in 2009 except for Ricky Nolasco, who was 11th. The pitcher that was in the top 10 in strikeouts per 9 innings that did not make the top 10 in AESR was Jonathan Sanchez. Last year, Sanchez only struck out about 30% more batters than the average pitcher, placing him 12th on the list.
The top pitcher on the list in 2009 was Justin Verlander. Verlander was effective at striking out a lot of batters. Of the 191 different batters he faced throughout the season, the struck out 121 of them at least once, but no batters more than 6 times throughout the year (Nick Punto and Mike Jacobs). Of the batters that Verlander struck out in 2009, he struck them out an average of 1.83 times more than expected.
Career
To place this in historical perspective, I have calculated the actual vs. expected ratio for pitchers from 1974 to 2009 that have pitched at least 2,000 innings. This will help place in context the greater significance of some of the more recent numbers shown above. The top 10 AESR’s for this time frame are:
1. Nolan Ryan - 1.744
2. Randy Johnson - 1.626
3. Pedro Martinez - 1.622
4. Roger Clemmons - 1.445
5. Curt Schilling - 1.398
6. David Cone - 1.349
7. Ron Guidry - 1.329
8. Tom Gordon - 1.328
9. Steve Carlton - 1.295
10. John Smoltz - 1.277
The top three pitchers in 2010 are off to an AESR start that rank in the top three, but what makes them seem less impressive is that the numbers shown above were accomplished over long careers. Nolan Ryan, for his career, struck out just under 75% more batters than the average pitcher during his career. The next closest pitcher, Randy Johnson, is almost 12 points behind.
There are several active pitchers on the list above, but none with long enough left in their careers to catch Nolan Ryan. Among active pitchers with significant time left in their careers, the closest is Kerry Wood at 48%. Given his current role as a closer, he could improve his numbers over the remainder of his career, but it would be next to impossible to even get close to Ryan. Rich Hardin is the next closest at about 46% over his career, but even he would have a long way to go to even challenge Ryan for the top spot in recent history. This is yet another reminder of just how special Ryan’s career really was.
Data Sources: www.retrosheet.org and www.baseball-reference.com
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