For some reason, the Rockies are a better team in April and September than they are in the other four months of the year. Since we have a lot to get to here, we'll jump right into the detailed proof: (The April and September records shown here also include a few games that spilled over into the last couple days of March or first couple days of October depending on how the calendar fell)
Since those final numbers are enormous, let's simplify them into something more familiar. To do this, I've taken the winning percentages each record amounts to and converted them into the corresponding records over a 162 game season. Here are those results:
Right away, those April and September records jump out as if to say, "Hey, the Rockies actually aren't that bad during those months." No, being a couple games under .500 isn't great, but it's a heck of a lot better baseball than they play in months like May or July. This is something I think many of us who watch the Rockies year after year after year subconsciously know, but seeing this in detailed form kind of drives it home for me, and of course, strengthens my curiosity as to why this happens.
My original hypothesis was that that it had to do with the pitching. My thinking here was that the Rockies did better in April because the pitching wasn't worn out yet, and then did better in September because of expanded rosters which would allow them to spread the pitching workload out more and put the best relievers exclusively in high leverage situations instead of having to mop up in other areas as well. This idea centered around the idea that Rockies arms have to throw more pitches than any other staff in the game and get worn down easier with the constant switch between throwing at altitude and throwing at sea level.
If this was true, it would have been interesting to explore how a 26 man roster (something that's not impossible in the future if MLB attrition stays at its current pace and bullpens continue to play a larger and larger role in every game) may help the Rockies more than any other club. We already see the Rockies carry an extra reliever at times to lesson the burden on the pitching staff as a whole.
So I started to dig and break down exactly where the Rockies were better in April and September, and while the data does reveal some interesting numbers, it also shows that my hypothesis doesn't hold water at all.
So here's some more tables. The first shows the Rockies' OPS by month, and the second shows the OPS of hitters against Rockies pitching (had to use OPS for both since I want to keep the metric consistent, and that's the most advanced metric widely available when breaking things down by month like this).
NOTE: I left out August and September of 1994 and April of 1995 altogether because the strike either completely or mostly wiped out those months.
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And since we need some sort of control element to compare how these numbers change from month to month (to guard against other potential natural rises in OPS because of things like weather, I made a third table calculating the average NL OPS for each month over this same time period. (Same rules apply to the months impacted by the 94/95 strike.)
That's a massive amount of data to take in when presented in huge blocks like this, so I took the most important information (the "Total" / average OPS of each month on the bottom of each table) and presented it in a more simplified form where it's easy to compare the finalized numbers side by side. Remember, the "Rockies Pitching" is actually the OPS against Rockies pitching, so higher numbers here mean they did worse.
In addition to those numbers, I also included a column showing the difference in the Rockies OPS each month and their opponents' OPS each month (written in red since the Rockies offense always came out on the negative side here).
Now we can start comparing some real meaning data. The first and rather obvious point to clear up is that the Rockies OPS and the OPS of opponents batting against their pitching is much higher than the NL average because of Coors Field. We knew that going in though. The point of information in that last column was to compare how the OPS changed month by month across the league to how it changed in Rockies related matters (More on that in a moment).
The second thing I immediately noticed is that the Rockies pitching does not allow a noticeably higher OPS in the middle four moths of the season (There went my hypothesis.) It's a shade higher than their average in June an July, but we also see that number as lower / better than average in May and August. In other words, the ups and downs of opponent success here seems to be more chaotic and random.
What we do see in the "Gap" column however are numbers pretty consistent with the overall team record by month, which we would expect. The OPS gap numbers are a little better in June than the overall record indicates, but they're in line for the most part.
The other thing that's extremely noticeable is that the months the Rockies do best offensively are also the months they are closest to .500 overall. For whatever reason, the Rockies have been much better at the plate in April, June and September than May, July and August. Trying to find a reasoning for that though is maddening. It could be any sheer number of things from having more home heavy schedules historically in the good offensive months (possible, but then it would also be hard to explain why opponents have their highest OPS in July) to just odd randomness.
One thing is certain however, the Rockies' offense has the largest month to month swing of any data we're looking at here. It also seems likely that they have the larger range than we see with most offenses. (I didn't get to every team, but I did smaller samples of several other clubs just for fun and the range wasn't as wide as it was with the Rockies)
To further illustrate what I'm talking about, here's one final table showing how much better (in green) or worse (in red) than average each group did within each month (This average is compared to each group's personal average. For instance: The Rockies overall franchise OPS is .784, so their .796 OPS in April means they are +0.012 for that month).
It's hard to say how much of this is sample size and how much this is switching from Coors to sea level, but it's pretty obvious that the change in month to month OPS across the league when you throw years an years of sample size on it is very minimal. We see a slight uptick in offense during the hot summer month of July and August, but everything tends to remain pretty close to that average league OPS over the last 23 years, which is .739.
Once you get to the pitching, we see a wider range of OPS totals. However, that number is still much smaller than the range we see with the offensive totals, which is the most inconsistent. The problem with this data however is that while it pretty clearly shows WHAT is happening, it does very little in terms of telling us WHY it's happening, which is of course just as if not more important. I would hypothesize that the Coors Hangover Effect is playing some sort of role here (that May number in particular looks mighty suspicious as the season starts to settle in), but it would take data I don't have to prove it since sites like baseball reference and fangraphs don't have features where you can double down on splits. Not only that, but once you get into data where you start splitting a split, you quickly run into sample size problems.
So when it comes to figuring out why the Rockies are better in April and September, we're left with more questions than answers. The biggest piece of potentially important information we did get here though is that it seems to have more to do with the offense than the pitching staff, which for me was honestly unexpected.
If you have any thoughts, I'd love to hear them in the comment section.