While monitoring the size and location of the strike zone this season on a monthly basis, of particular interest to me has been the resulting run environment across major league baseball. Runs have dried up in recent years as the strike zone has expanded, and some of my previous work has estimated how much of the dip in run scoring is directly attributable to the larger zone.

This relationship has remained largely consistent over the past half-decade or more. As the bottom of the zone continues to fall, runs and walks drop along with it, while strikeouts creep higher. It is with this as a backdrop that we come to the month of August 2015. Consider the following visual of average runs scored per team per game by month over the past five seasons:

After three full years elapsed without teams averaging as many as 4.30 runs per game over a single calendar month, major league teams averaged a whopping 4.49 runs per game in the just-completed month of August. This may not sound like a big difference, but you can see from the image above just how unprecedented this is in recent years.

The high octane month has powered the yearly offensive numbers to a point where run scoring in 2015 is now almost assured to be higher than in 2014. Here are the latest data:

RECENT STRIKE ZONE HISTORY Year Strike Zone Size (sq. in) Strike Zone Size Below 21” (sq. in) K% BB% R/G 2009 435 0 18.0% 8.9% 4.61 2010 436 6 18.5% 8.5% 4.38 2011 448 11 18.6% 8.1% 4.28 2012 456 19 19.8% 8.0% 4.32 2013 459 30 19.9% 7.9% 4.17 2014 475 47 20.4% 7.6% 4.07 2015* 478 50 20.2% 7.5% 4.20 * At the end of August 2015

Here are the accompanying visualizations of the strike zones to date in 2015, from the umpire’s perspective:

While this August was unusually productive for offenses around the league, as part of my last strike zone update at the end of July I noted that at least in recent seasons, September tends to play out with notably fewer runs scored than the season average. Here’s a table that shows the differences:

(SEPTEMBER – FULL SEASON) DELTAS Year K% BB% R/G 2009 0.3% 0.1% -0.11 2010 0.8% 0.0% -0.17 2011 0.8% 0.1% 0.13 2012 0.7% 0.1% -0.09 2013 0.8% 0.1% -0.12 2014 0.3% -0.4% -0.16

I wanted to determine why September has been less kind to offenses than other months. Given the extreme outburst of runs in the August that just wrapped up, the analysis can serve the secondary purpose of attempting to understand why that was the case.

Let’s consider possibilities one at a time.

Temperature

Could it be as simple as the typically cooler weather in September cooling off bats? It has been shown that higher temperatures allow fly balls to travel farther, logically leading to more home runs, which sounds like a good way to score more runs (unless you think home runs are rally killers!).

Here is an image showing average runs scored per team per game against average monthly temperature over the past five seasons:

While there are certainly some higher-scoring hot months and somewhat lower-scoring cool months, on the whole there is virtually no relationship between monthly temperatures and runs. Even if temperature did play a telling role in runs scored per game, on average both April and May have been notably colder months than September. August 2015 was the fourth-warmest month of the past five seasons, for what it’s worth.

“Luck”

We already know that strikeouts have been up in September. Could it be that when hitters have been making contact in the final month of the season they’ve also been less fortunate than earlier in the year when pitching and defense are still being fine-tuned?

Here’s a look at batting average on balls in play (BABIP) over our five-year period of study, by month:

From the image we see that BABIP plays no role in the variation in runs scored per game from month-to-month. Even if it was impactful, it turns out that BABIP is typically lowest in the first half of the season, with July, August and September all having higher BABIPs that April, May and June.

While it doesn’t stick out, August 2015 did have the second-highest BABIP of any month over the last five seasons, at .302. In fact, June through August 2015 comprise three of the six highest BABIPs by month since 2011. It’s interesting to note that despite defensive shift totals setting new records year after year, defensive efficiency across the major leagues is certainly not getting any better on the whole.

Another metric that we can lump under the “luck” category is home runs per fly ball.

It would appear we have made an impactful discovery; HR/FB percentage drives a notable amount of the monthly variation in runs scored per game. September has seen the second lowest HR/FB percentage of any month in the season over the past five years, higher only than April.

This also goes a long way to explaining the anomalous month of September 2011 in the table above, when run scoring was actually higher than the season average for the only time in the PITCHf/x era. September 2011 was unusual in that it saw a HR/FB percentage that was the second highest monthly rate of that season.

Finally, the runs-aplenty month of August 2015 can be seen in the top right corner with the highest HR/FB rate of the last five years by a fairly wide margin. From this analysis it is apparent that when batters made contact last month, they fared better than normal both as far as reaching base when the ball stayed in the park and having the ball exit the yard when it was put in the air.

Out of curiosity, I checked the correlation between average monthly temperature and HR/FB percentage, and found just a small correlation (R-squared of 0.04). In reality the more complete effect on a baseball in flight would be air density, in which temperature plays just a part, along with elevation, air pressure and relative humidity. Perhaps if the other elements of air density were readily available, a study of runs scored per game vs. air density would prove more fruitful than purely temperature.

Pitch Velocity

We know that pitch velocity is up. Here is a look at monthly runs per game and average four-seam fastball velocity over the past five seasons:

There is a visible correlation, although in this case I would not say necessarily without further study that this is causation as well. Velocities have been increasing at the same time as the strike zone has been falling, and the latter may be much more of the cause of this trend than the former.

Velocities are certainly higher from June onward in the season, but July and August see higher speeds than September, so regardless this relationship doesn’t go too far toward explaining the lack of offense in September.

Nonetheless, a look at velocities by month shows that August 2015 is an outlier in the top right corner here once again; Four-seam fastballs were a full 0.2 mph faster on average than any other month in the last half decade. Another startling way to break this down is to note that before last month, teams scored on average fewer than 4.20 runs per game in all 11 months where the average fastball velocity was over 92 mph. Then all of a sudden last month four-seamers were thrown harder than ever, and teams scored almost 4.50 runs per game. What a crazy month!

Strike Zone Size / Average Pitch Height

The method that I use for measuring strike zone size is not ideal for measuring on a month-by-month basis; sample sizes of two months or greater give far more accurate results. Given that we know during this five-year stretch the strike zone growth was overwhelmingly being driven by a falling bottom of the zone, we can make use of a proxy for zone size, borrowing an idea from Rob Arthur.

The proxy is to look at the average height of all pitches thrown on a monthly basis during our period of study. Pitchers adapt quickly when the strike zone is being called lower, and throw more frequently down in the zone.

The lower that pitchers have been allowed to throw and get away with it via positive reinforcement from umpires, the better it has been for run prevention. Except, of course, for August 2015, which once again stands out, only this time in the top left corner. Pitchers kept the ball lower last month than any month in the past five seasons, yet teams scored more often than in any of those same months. Is this really just a “lucky” month for offenses where they benefited from better than expected results when making contact? Or could this, along with the noted streak of months with slightly better BABIP results, be the start of a sign that teams and batters have started adjusting to the new era of more strikeouts and a lower strike zone?

On the whole, there is very little difference in average pitch height among the months of the season, so while this is a factor, it is really explaining why offense has been down on a year-over-year basis more so than a month-to-month basis within a season.

Batter-Pitcher Familiarity

We know that the Times Through The Order (TTOP) penalty exists: Batters in the aggregate perform better each subsequent time they face the same pitcher in the same game. This effect may be due to the pitcher tiring, the batter getting familiar with the pitcher’s arsenal, timing, and release point, or some combination of those things and other aspects of the game.

We know that September is a month where active rosters can exceed the 25-player limit that is in place from April through August. This means that most teams add call-ups, some of whom are debuting in the majors.

I wondered if the TTOP extended to the career level. In other words, on the whole do batters gain an advantage after having faced a particular pitcher many times compared to those first few looks? Anecdotally, it feels like often rookie pitchers baffle major league lineups the first time through, when advance scouting reports have to try to fill the void of personal experience for big league hitters.

To examine this question, I looked at wOBA by major league career batter-pitcher times faced. The sample for this was all plate appearances since 2011 where the first batter-pitcher match-up at the major league level occurred no earlier than 2011. (Actually, I know only that each batter-pitcher match-up did not occur between 2007 and 2010, so it is possible a few pairings faced each other five years or more prior). Of course this is not perfect; hitters may have faced pitchers in spring training, in minor league ball or even college or high school. But I feel it is a close enough approximation to address this question.

Here are the results of wOBA by batter-pitcher times faced at the major league level:

There certainly appears to be a trend suggesting pitcher familiarity is advantageous to batters not just within a game, but at the career level. Of course some of this trend is in fact just the TTOP, but this extends to multiple games over even multiple seasons against both starting pitchers and relievers.

It is important to understand that the graph above includes results from all batter-pitcher match-ups. Less-talented, fringe major league hitters and pitchers would tend to make nearly all of their major league match-ups on the far left of the graph, since they wouldn’t stick around in the majors long enough to face the same opposing player multiple times. Conversely, batter-pitcher match-ups that number in double digits likely suggest that both the batter and pitcher are talented enough to have survived in major league baseball long enough to have faced the same opposing player on so many occasions.

Another visualization that agrees with the fact that batters gain an advantage from additional looks throughout a career is this one that offers strikeout rate:

Some of the advantage gained by hitters may be due to the fact that as pitchers age, velocity wanes. Conversely, plate discipline for batters generally holds steady or can improve as they age.

It also lends credence to the move to relief pitcher specialization. More pitchers throwing fewer innings means pitchers can throw harder and potentially have to face the same hitters fewer times over a career, both seen as positives by this study.

Are there really more major league debuts in September than other months? As far as pitcher debuts go, April and June have more on average than September. For hitters, on average there are more debuts in only June than September. There is some logic to April, June and September being the most common months of the season for players to debut. April debuts would be players who make the team with solid spring training performances, June debuts would be prospects held down until the Super-Two deadline passes, and September debuts would be call-ups added when rosters expand. Given that June and September have been the lowest scoring months of the past five seasons, perhaps we’re on to something!

Unfortunately, when we look at the average batter-pitcher time faced for all plate appearances over each month of the season, the relationship doesn’t hold water for September:

BATTER-PITCHER FAMILIARITY Month Average Batter-Pitcher Time Faced* April 4.02 May 3.61 June 3.60 July 3.97 August 3.70 September 4.41 * Of all PA where first batter-pitcher match-up in the majors occurred 2011-2015.

September actually has the highest average time faced of any month within the sample used for this study. Of course there are competing interests here; while many new players are added to rosters in September, players who have been on big league rosters all season have now had April through August to amass more plate appearances against rival pitchers. The sample again also excludes all plate appearances where the batter and pitcher faced one another in the majors before 2011, so the calculated average is incomplete insofar as it does not include these battles of more senior players.

It also must be considered that teams still in contention are likely to stick with their most productive players down the stretch, and these players are often “veterans” who “have been there before.” These players would tend to have faced major league pitchers a number of times each, in particular with all of the intra-division play that occurs in the final weeks of the season.

Talent Level

One final theory to the lower run scoring environment in September is that as stakes are higher for teams in playoff contention, managers may decide to play most talented players on a daily basis. Similarly, the bullpen may get shorter, with the top relievers of the year getting a higher percentage share of innings pitched than they had earlier in the season.

This would have to be studied by using preseason projections of a fairly all-encompassing metric for all players (say wOBA for batters, wOBA allowed for pitchers, for example). With these in hand as proxies for “talent level,” the average of these metrics could be calculated by month and then a relationship with runs scored per game could be explored.

Alas, I could not easily find projections for one all-encompassing metric for both batters and pitchers from one projection system for all years 2011 through 2015. So this last theory is left as an exercise for the reader.

Putting It All Together

I would make the case that the recently concluded month of August was the most unusual month of baseball in the past five seasons. Pitchers threw harder than ever, to a strike zone that was lower than ever, yet batters responded by hitting home runs on a higher percentage of fly balls than any other month in the past five years and a compiling a higher BABIP than all but one month over that time period. The result was the highest scoring month since the strike zone began to fall.

The question of why September is a dry month for offenses hasn’t fully been answered, but perhaps some headway was made. Home run per fly ball percentage tends to be lower in September than the yearly average, and this behavior appears to drive a fairly significant amount of the variation in monthly runs scored per game. Fastball velocities are slightly higher than average, which on the whole helps to suppress offense. There are more player debuts than normal in September with the expanded rosters, and initial batter-pitcher match-ups favor the pitcher.

Yet it it inconclusive whether in the aggregate September really led to more of these match-ups where the batter was relatively inexperienced with the hurler on the mound than in other months of the season, given that most hitters had gathered more experience with the set of major league pitchers over the season. A further study of air density and player talent level by month would be interesting to uncover other potential theories not addressed in this study.

References & Resources