Walks are undesirable outcomes. As much as the opinions of the fantastic readers here vary, I don't think anybody would argue that walks -- in general -- are a good thing. There is, however, some question about just how bad walks are, particularly for Rockies pitchers. It could be that walks aren't as punishing at Coors Field as they are elsewhere, which means the Rockies should emphasize strikeouts instead. It's also possible that walks are more punishing at Coors Field and thus should receive extra emphasis. Or perhaps there's no difference at all and this is much ado about nothing. This leads us to two interesting questions:
1) Is limiting walks more important than increasing strikeouts for Rockies pitchers?
2) Is limiting walks more important for the Rockies than it is for other teams?
To answer the first question, we can look at the correlation of walk rate to ERA for Rockies pitchers and compare that to the correlation of strikeout rate to ERA. We'll look at the Rockies' entire history to give us the largest possible sample size.
Before we get started, it's important to add the disclaimer that ERA isn't a perfect stat. Even over a full season, an unusual amount of luck can make a team's ERA look better or worse than it actually should've been. However, team ERA over a full season will generally be pretty close to a team's overall talent level, and even if there are a few outliers, it should still give us a decent baseline.
Another important aspect of this is that the league climate has changed quite a bit since 1993. Today, a pitcher who strikes out six batters per nine innings (SO/9) is considered a low strikeout pitcher. However, in 1993, 6 SO/9 would've been slightly better than average! Similar differences can be seen in ERA and walk rates. To combat this, I used a simple conversion that adjusts for league climate so that, instead of looking at the raw strikeout and walk rates, we're looking at them relative to that season's league average (FanGraphs already does this with their ERA- statistic, so that work has been done for me). The formulas are as follows:
(Team BB/9 / league BB/9)*100 = BB/9-
(Team SO/9 / league SO/9)*100 = SO/9+
For both stats, 100 is league average, with lower being better in the case of BB/9- and higher being better for SO/9+. Now that we've defined our experiment, let's dive in.
First, we'll look at the relationship between SO/9+ and ERA-. If there is a strong relationship between the two, we should expect to see ERA- go down as SO/9+ goes up. Here are the numbers in both table and graphical form:
There does not appear to be a clear relationship here. Sure, there are a few years like 2000 and 2007 when the numbers matched almost identically, but for the most part there's nothing here. More than a third of the time, the Rockies' SO/9+ and ERA- haven't even been within 20 points of each other! That isn't the only important thing, though. As mentioned before, if there was a strong relationship between these two things, we would see a negative correlation, meaning that ERA- would go down as K/9+ went up.
Instead, what we see is a graph of two lines that fluctuate independently. The correlation between these two comes out to -0.16, which is in fact negative, but it's very weak. Now, the question becomes whether or not we see a correlation between BB/9- and ERA-.
Since lower is better for both BB/9- and ERA-, we should expect them to move hand in hand if there's a relationship between the two. As BB/9- goes down, ERA- should follow and vice versa. Let's check out both the table and the graph of this relationship:
Now that is what a dependent relationship looks like. In nearly half (11 out of 23) of the seasons in Rockies history, BB/9- and ERA- have been within five points of each other. Never in club history has the difference even approached 20 points, something that has happened eight times between SO/9+ and ERA-. Notice on the graph that the two lines flow together in unison throughout (there's even a period between 2007-09 when the two lines are nearly indistinguishable from one another), which is exactly what you're looking for in a positively correlated relationship. The correlation between the two comes out to 0.85. In other words, there's a strong positive correlation.
Before we go any further, it's important to note that there are exceptions to every rule. Ubaldo Jimenez and Jorge De La Rosa, for example, have both had plenty of success with the Rockies despite higher than average walk rates, so it isn't as if the Rockies should write guys like that off altogether. However, given this data as a whole, I think we can say pretty conclusively that the answer to our first question is, yes, limiting walks is more important for Rockies pitchers than increasing strikeouts.
Our first question now has an answer, which means that it's time to turn our attention to our second question. We know that walks are more important than strikeouts for Rockies pitchers, but are walks even more important for the Rockies than they are for the rest of the league? We can examine this the same way we did with the Rockies. We're looking for a positive correlation, which would look like a graph in which the two lines move up and down together. Let's take a look at the other 29 teams in the league from 2013-15:
Well, we've definitely got a relationship here. That isn't surprising; we already know that there's a relationship between walks and allowing runs, and we also know how strong the relationship is for the Rockies. What we want to know is whether the relationship is as strong as the Rockies' relationship to walks. Let's see what we can find out from this information.
The rest of the league has had a SO/9- within five points of their ERA- in 34 out of 87 seasons. That's just 39 percent of the time, compared to 48 percent of the time for the Rockies. We also see eight instances in which the difference between SO/9- and ERA- was greater than even the biggest difference the Rockies have had in club history. We can also see some pretty wide gaps in the graphs mixed in with lines that do mostly flow together. The correlations of these three years, respectively, are 0.52, 0.41, and 0.61. These are all moderate correlations and they all show that every team should desire pitchers who limit walks, but none of those numbers approach the strength of the Rockies' correlation. It looks like the answer to our second question is also yes: Limiting walks is even more important for the Rockies than it is for the rest of the league.
So, not only is limiting walks more important than increasing strikeouts for the Rockies, it appears to be even more important for them than it is for the rest of the league! Knowing this makes it even more unacceptable that the Rockies led the Majors in walks in 2015. On the bright side, the Rockies designating high walk pitchers John Axford, Tommy Kahnle, and Rex Brothers for assignment over the weekend seems to show they realize that pitchers who walk a lot of guys just don't work for them. The next step would seem to be focusing on acquiring and/or holding on to pitchers who have good command.
I'm a firm believer that -- provided the pitcher believes he can succeed -- pitchers of all types can do well at Coors Field. However, if there is a secret formula for success, the main ingredient of that formula is not putting people on base.