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Explaining Bob Gibson's 1968 Season

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Most fans know that Gibson had an incredibly low ERA of 1.12 in 1968. Even considering that the league ERA was just 2.98 that year, what he did is still great. What explains this performance? Did Gibson have great "stuff" that year? Was he lucky? Or was it a combination of luck and skill? If so, how much of each?

Luck is sometimes a factor in baseball. Some years a guy hits well with runners in scoring position (RISP), some years he doesn't. In 2004, A.J. Pierzynski, for example, hit .272 overall but .307 with RISP. In 2005, he hit .257 overall but only .236 with RISP. Its not likely he forgot how to hit with RISP all of the sudden. For pitchers, the batting average they allow on balls in play may be out of their hands (as pointed out by Voros McCracken). A pitcher might get lucky on balls in play one year, with more getting caught than normal (or his fielders might be especially good one year). Is this what happened to Gibson?

To test this, I ran a regression in which a pitcher's ERA was the dependent variable and his strikeouts, walks and HRs allowed per 9 IP were the independent variables. I used the regression equation to predict each pitcher's ERA then found out how much it differed from his actual ERA. If a pitcher had an ERA lower than what his strikeouts, walks and HRs allowed per 9 IP predicted, he most likely gave up fewer hits on balls in play than average. Here is the regression equation

(1) ERA = 2.19 + 1.436*HR - .159*SO + .303*BB

Again, all stats are per 9 IP. BB includes both walks and HBP. The data includes all pitchers who qualified for the ERA title from 1963-68 (I used this period since it was an especially low scoring period). There were 420 pitchers. The r-squared was .548, meaning that 54.8% of the variation in ERA across pitchers is explained by the equation. The standard error is .469.

Plugging Gibson's 1968 data into equation (1) leaves an ERA of 2.02. That is a very large 0.90 above his actual ERA of 1.12. So it appears that he must have done especially well on balls in play (more on this later). The table below shows the leaders in how much lower their predicted ERA was than actual their ERA.

Gibson is not first in "Diff" but he did have a big one at #8 (something interesting may be going on with the White Sox, with John, Horlen and Fisher all being up there). The table below shows the 25 lowest predicted ERAs.

Notice that Gibson's 1968 season, although the best, does not dominate the way his actual ERA dominates. The next table shows the lowest 25 actual ERAs from the period.

In predicted ERA, there are 24 pitchers within .40 or less of Gibson. But in actual ERA, it is only one!. So using only pitcher determined outcomes (strikeouts, walks and HRs allowed), brings Gibson back down to earth. He is the leader, but he is not so far away from the rest of the pitchers.

Now that we have seen these results, lets check to see if Gibson did indeed have a low batting average allowed on balls in play (BABIP) in 1968. The table below shows Gibson's BABIP for each year of his career along with the BABIP of the entire Cardinal staff (including Gibson). Notice that his lowest BABIP was in 1968 as well as the biggest difference from the Card's staff.

In some years Gibson had a lower BABIP than the Cards staff, in other years, higher. But he definitely had a low BABIP in 1968 (some of the numbers in the "Diff" column may look slightly wrong due to rounding). The next table shows the lowest 25 BABIPs of the period.

Gibson did not have the lowest BABIP, but he was #18.

If we try to predict Gibson's ERA using regression analysis and also include hits on balls in play, we will be able to predict his ERA much more accurately. I ran a regression in which a pitcher's ERA was the dependent variable and his non-HR hits, walks and HRs allowed per 9 IP were the independent variables (since I am using what happens on balls in play here, it is not necessary to put strikeouts in-every strikeout means one less chance for a hit and the number of hits is already accounted for in the model).

(2) ERA = -2.17 + 1.397*HR + .466*NONHR + .310*BB

The r-squared was .811, meaning that 81.1% of the variation in ERA across pitchers is explained by the equation. The standard error is .303. I then predicted each pitcher's ERA using equation (2) and found how much that differed from their actual ERA. It predicted Gibson to have a 1.49 ERA. This is only .37 above his actual ERA, much more accurate than equation (1), which was off by .90. But the point here is not to find which equation is most accurate. The point is that once you include what happens on balls in play, we get a much more accurate picture of Gibson's performance. And in this case Gibson was off by just 1.22 standard errors (.37/.303) while he was off by 1.92 standard errors with equation (1) (.90/.469). This supports the thesis that Gibson was helped quite a bit by his low BABIP.

A few weeks ago a I posted an article about the best seasons in something called "Fielding Independent ERA" or FIP ERA (see sources at the end of this article). In that article I used a more sophisticated approach than I used here. The lowest 25 FIP ERAs of this period are in the table below.

Notice that Gibson is only second (the FIP ERA's do not completely correspond to predicted or actual ERAs mentioned earlier since all ERAs in the FIP ERA study are normalized to a league with an ERA of about 3.70). The FIP ERAs here are also different because HRs allowed were adjusted for park effects, something not done for the above analysis.

We can tell from the following stats that Gibson's performance in 1968 was not as far above his other seasons as ERAs alone would indicate. The table below shows his strikeouts, walks and HRs allowed per batter faced for each year in his career with 100 or more IP. 1968 is clearly his best, but some other seasons rival it. In 1970, for example, his strikeout and HR rates are very close to 1968. In 1967, his strikeout rate and BB rate were similar to that of 1968. But 1970 was a much higher scoring season than 1968 with the NL ERA being 4.05. Gibson's fielding independent stats in 1970 are almost as good as they were in 1968.

The next table shows his FIP ERAs for the years 1961-1974 (season when he had at least 150 IP).

As mentioned earlier, these FIP ERAs are all normalized for a league with about a 3.70 ERA. According to this, Gibson was better in 1970 than in 1968. That is, taking park effects into account to adjust HRs, using only pitcher controlled stats and comparing to the league average shows his 1970 season to be even better. This, too, suggests that 1968 was helped quite a bit by a very low BABIP. The big difference between the two seasons was his BABIP of .230 in 1968 and his .299 BABIP in 1970. The 1968 BABIP was far below the team BABIP while his 1970 BABIP was above the team BABIP.

I also broke down his performance into RISP and non-RISP situations to try to understand how his ERA could have been so low. He allowed a batting average of .184 overall but just .141 with RISP (and .193 in non-RISP situations). To see if this made any difference, I ran a regression with ERA as the dependent variable and on-base percentage (OBP) and slugging percentage (SLG) were the independent variables. That predicted Gibson to have an ERA of 1.25 (just using pitchers from 1968). Then I broke down OBP and SLG into RISP and non-RISP situations. The resulting regression equation predicted Gibson to have an ERA of 1.13. So his RISP performance also helped a little in making his ERA so low since the regression that took RISP into account was more accurate. His career average allowed overall was .228 while with RISP it was .219. Those two are pretty close, indicating that Gibson probably did not have any special ability with RISP. He just happened to do very well in those situations in 1968.

I also took the natural log of ERA in one of the regressions in case ERA had a non-linear relationship with the other stats. Doing this did not improve the results.

There is one other issue with the fielding in 1968. It appears that Gibson got a little lucky in 1968 with more balls in play than average being turned into outs. Perhaps the fielders were playing better or trying harder behind Gibson that year than they were for the other Cardinal pitchers. But Gibson actually gave up more unearned runs than would be expected as compared to the entire Cardinal staff. The staff ERA was 2.49. But if we include all runs, Gibson gave up 1.45 runs per 9 IP while it was 2.87 for the whole staff. Gibson's runs per 9 IP was 29.46% above his ERA while for the whole staff it was 15.26% higher. So Gibson was hurt more by unearned runs than the whole team, indicating that his fielders hurt him more than other Cardinal pitchers. Perhaps they were hustling more and simply got to more balls, leading to more errors. Maybe official scorers called more errors than expected to protect Gibson's ERA (it is true that Gibson's runs per 9 IP is .33 higher than his ERA while it is .38 for the whole staff, possibly showing that Gibson was hurt less by unearned runs-but with fewer base runners allowed, any given error should have hurt him less so a .33 increase is proportionally worse for him than a .38 increase was for the whole team).

Sources:

The Complete Baseball Encyclopedia from Lee Sinins

The Best Fielding Independent Pitching Seasons From 1920-2005
http://www.beyondtheboxscore.com/story/2006/5/23/143235/740