In the lab: Checking in on Bases Per Out

One of the stats I have championed here on this site is bases per out. Naturally, much like sports talk radio, we are getting new readers every day. So, every once in awhile it makes sense to retrace our steps and reintroduce the whys and what fors of the metric. The idea behind bases per out is to get an accurate metric that measures offensive effectiveness. Obviously, since most statistics are mirror images it also measures pitching and defensive effectiveness as well.

The formula for bases per out is fairly simple and rudimentary. It combines total bases, walks, stolen bases, and hit by pitches and divides it by the total number of outs. Of course, when one has only a calculator and spreadsheet there are some things that get missed with the simple number. All numbers must start with a frame of reference. The raw BPO numbers do not account for the league average BPO or for individual ballpark effects.

Obviously, we can add those back in after the fact. The common source for these numbers is Baseball Reference. Unfortunately, they do not includes bases per out as an official stat. I suppose that could make me a bit of a pioneer in that regard. I have tried to lobby them to include, but that has fallen on deaf ears. So, this is an introduction to bases per out. We will start with the global and move back to the specific.

When sites like baseball reference calculate numbers like OPS+, they start with the league average OPS and them move onto ballpark effects. We will include both here. To start, we will look at the major league average for BPO in 2025 and 2026. We do that to show change over time. Obviously, current numbers are almost always based on past norms. We need to know what is currently happening in real time. The numbers below are accurate coming into Sunday’s action.

MLB Average BPO

Now comes the hard part. One of the cardinal rules of all social science is that we cannot attribute causation in human endeavors. What we can do is assert correlative relationships. The first step to identify systemic reasons for the change is to note any differences. We know that the ABS system is the major rule change between 2025 and 2026. Offensive numbers would appear to be down, so the first and most likely hypothesis is that the ABS system is favoring pitchers.

Naturally, this would cause us to look at the actual records to see if more strikes were added than taken away through the ABS system. Yet, those numbers would only say so much. The problem is that umpires have also changed their strikezones to adjust. So, we would need to look at total strikes and total balls called and where in the zone those are happening. Obviously, we are not going to get that far into the weeds.

When we eliminate ABS as a possibility then we are left with more minor possibilities, but the most likely other explanation is simple random distribution. We can track this as the year goes on and see if the results from 2026 come closer to 2025. The long and short of it is that the improvements in the Astros offense might be more pronounced given these results. It also means the losses in their pitching are more magnified.

However, we should move onto the second part of equation. This is where we look at ballpark effects. Is Daikan Park a hitters or pitcher’s park? As you might imagine, the answer is not necessarily cut and dried. Just like the overall numbers, the numbers for individual ballparks can fluctuate from season to season as well. Average ballparks score 100 on the form with hitters parks over 100 and pitchers’ parks below 100.

Park Factors

So, Daikan park ends up being more or less neutral when it comes to scoring runs. However, it is playing with a little more offensive output this season. Like anything else, that is a mirror image of it each other. We can apply a simple multiplier to give each player a BPO+. The overall park factor (PARK) is the one that we will use calculate our BPO+.

Some of you might be wondering why we go through this exercise in the first place. Why do we need to jump through all of these hoops? The quick answer to that question is that light is the best disinfectant. We teach our students to always show their work. Hopefully, these numbers will make more sense if we explain the science and math behind them.

In the coming days we will be taking a look at individual position players and pitchers. We will calculate not only their BPO, but also their BPO+ based on the numbers we see above. Most of the time these numbers reflect what we already know, but occasionally we miss something. I know the basics may not be exciting, but understanding the basics helps us break down individual performance in more detail.