Yesterday, the Mariners traded for Mark Trumbo. Of course they did. More than any other franchise in baseball, the Mariners have a history of seeing value in bat-only sluggers, often surrendering defensive value to try and build a line-up that can conquer Safeco Field. I’ve been predicting the Mariners would trade for Trumbo all year, and with the Diamondbacks finally admitting that they needed to clear up their roster logjam, this was one of the easiest deals to see coming.

Of course, the fact that the Mariners have been acquiring players exactly like Trumbo for years, and never really having much success in doing so — even though the the acquisition of Nelson Cruz has gone as well as could possibly be imagined so far, the team’s position players are still 22nd in wRC+ and 28th in runs scored — makes it easy to point out that Trumbo is more of what the Mariners already have, but not at all what they lack. The Mariners have a .298 on-base percentage as a team, and they just acquired a guy who has a .299 OBP this year (while playing in hitter-friendly Arizona) and a .298 OBP for his career. This factoid was pointed out by a litany of people on Twitter in the aftermath of the trade.

Would feel better about Mariners if they got AJ Pollock, not Trumbo. Their problem is being 14th in AL in OBP, not 9th in slugging — Peter Gammons (@pgammo) June 4, 2015

The team with the second-worst OBP (.298) in the American League just acquired a hitter with a .298 career OBP/.299 OBP this season. — keithlaw (@keithlaw) June 4, 2015

Hey, another low OBP guy! Problem solved! https://t.co/m8fNiJX4pV — David Schoenfield (@dschoenfield) June 3, 2015

Schoenfield expanded on his tweet in a Sweetspot blog

Look, I don’t know who you blame for the franchise’s 15-year inability to develop hitters. But you can certainly blame Zduriencik for not understanding that you score runs by getting guys on base and not making outs. It’s about OBP.

On-Base Percentage has been a cause célèbre for statistically-inclined writers for the last three or four decades, and as analytical concepts and ideas have become adopted into the mainstream, OBP has become one of the primary tools used to evaluate a hitter’s performance. And by OBP, Mark Trumbo sucks. There’s no two ways around that; if you value out-avoidance, you probably don’t think very much of Mark Trumbo. He is exactly the kind of player who was wildly overrated by MLB teams 10 or 20 years ago, but as this trade shows, Trumbo was now acquirable for a platter of non-impact prospects having bad years. There wasn’t a huge demand from other teams to bring Trumbo into the fold, because a bat-only player who makes outs 70% of the time just isn’t all that great.

But at the same time, there’s a pretty big gap between arguing that teams should place real value on OBP and arguing that teams should either primarily or only value hitters by their OBP. OBP is significantly more valuable than SLG — which is why we use wOBA/wRC+ and not OPS/OPS+ around here, since linear weights models correct that issue — but SLG does matter, and low-OBP sluggers can be effective offensive performers even while they make a lot of outs. Even with that .299 OBP in Arizona this season, Trumbo’s running a 115 wRC+, making him a more-than-respectable offensive performer.

But the argument that Schoenfield (and others) are making isn’t a linear-weights based argument. It’s that the team already had power but lacked OBP, so if they were going to upgrade their offense, they needed to improve their OBP and not their SLG. Strengthen your weakness, in other words, rather than reinforcing the thing you’re already good at. The problem is that this idea is actually just wrong.

If you want to evaluate a player’s impact on a specific team’s offensive production given the construction of the rest of their roster, the best tool to use is called a Markov Chain. Tom Tango and John Beamer laid a lot of the groundwork for using Markov Chains as baseball run estimators about a decade ago, and then a couple of years ago, Steve Staude followed up on their work in a series of posts here on the site.

If you’re into the nuts and bolts of how Markov Chains work, I highly encourage you to read all those posts, but I know many of you are probably just here for the conclusion, and helpfully, Staude actually used Trumbo as an example of what would happen if you put either a low-OBP slugger or a high-OBP slap hitter into different types of offenses. So rather than just repeat his work, I’m just going to quote liberally from Staude’s piece “Team-Specific Hitter Values by Markov”. First, here’s a slightly cleaned up version of the table he included showing the differences in run expectancy for adding either Trumbo or Alberto Callaspo — then a high-OBP guy with minimal power — to different kinds of offenses.

Season Team or Player OBP SLG Markov (tweaked) Markov (default) BaseRuns Runs Created 2011 Mark Trumbo 0.291 0.477 4.44 4.77 4.83 5.07 2011 Alberto Callaspo 0.366 0.375 4.99 5.21 5.13 5.22 1963 Colt .45’s 0.283 0.301 2.84 2.77 2.92 2.96 1963 Colt .45’s+T 0.284 0.318 3.00 2.98 3.12 3.16 1963 Colt .45’s+C 0.292 0.308 3.02 2.98 3.11 3.16 1965 Mets 0.277 0.327 2.96 2.97 3.12 3.15 1965 Mets+T 0.278 0.342 3.19 3.14 3.29 3.33 1965 Mets+C 0.286 0.332 3.22 3.15 3.29 3.34 1968 Mets 0.281 0.315 2.95 2.85 3.04 3.11 1968 Mets+T 0.282 0.331 3.09 3.04 3.21 3.29 1968 Mets+C 0.290 0.321 3.12 3.04 3.22 3.30 2011 Mariners 0.292 0.348 3.45 3.39 3.54 3.61 2011 Mariners+T 0.292 0.361 3.55 3.53 3.67 3.75 2011 Mariners+C 0.300 0.351 3.59 3.54 3.68 3.76 1994 Yankees 0.374 0.462 5.90 6.52 6.40 6.63 1994 Yankees+T 0.364 0.464 5.66 6.23 6.16 6.43 1994 Yankees+C 0.373 0.450 5.77 6.33 6.22 6.42 1996 Mariners 0.366 0.484 6.10 6.53 6.45 6.77 1996 Mariners+T 0.360 0.483 5.91 6.33 6.28 6.60 1996 Mariners+C 0.366 0.473 5.99 6.40 6.32 6.61 1999 Indians 0.373 0.467 6.12 6.55 6.45 6.69 1999 Indians+T 0.366 0.468 5.93 6.34 6.28 6.54 1999 Indians+C 0.373 0.457 6.01 6.41 6.32 6.54

And here’s what Staude wrote after he showed that table.

So, at the top two spots on the list, we have the theoretical runs scored of teams full of clones of either Trumbo or Callaspo. This is basically the same idea as the RC27 you can find amongst ESPN.com’s sabermetric stats (which places Trumbo at 4.47 and Callaspo at 5.22, by the way). You can see right away that the Markovs favor Callaspo over Trumbo more than you might expect from their wOBAs and wRC+. Do you remember seeing the exponential growth curve of runs depending on team OBP in my last article? That explains why this is the case — it’s an important team effect that wOBA doesn’t try to account for. You’ll also notice that relative to Trumbo, Callaspo is worth a lot more to the good offenses than to the bad ones. In particular he’s worth more to the high-OBP teams, as besides the exponential impact his better OBP has on runs, his relative lack of power hurts less. That’s because the value of a single to a high-OBP team is greater than it is to a low-OBP team, especially relative to a HR (see the graphs in my second article if that confuses you).

I’m going to quote one more paragraph from another of Staude’s posts, because he lays out the issue pretty well:

When you think about it, the closer to 1.000 a team’s OBP becomes — at least past a certain point — the closer to 1 the value of any on-base event should be. If the bases are always loaded, then even a walk is always going to drive in a run, and you’re always going to be driven in by somebody behind you. Sure, a home run with the bases loaded is going to drive in 4 runs, but then you have to consider that the hitters behind the home run hitter would have driven in the runs anyway. Basically, it’s a communist utopia of hitters, where all varieties of hits, and even walks, have equal worth.

As a team’s overall OBP goes up, the relative value of SLG goes down, because you don’t need one big hit to drive in a bunch of runs when there’s a decent likelihood of stringing a bunch of smaller hits (or walks) together. And importantly, the inverse is also true; as a team’s OBP goes down, the relative value of SLG goes up, because singles and walks to a bad offensive club are less likely to score runs than a guy hitting a ball over the wall.

In other words, a team of low-OBP sluggers will actually draw a larger benefit than linear weights suggests from adding another low-OBP slugger to the mix than they would adding a high-OBP slap-hitter. If you already have a team that makes a bunch of outs, and you have to choose between two equally valuable hitters — one of whom is a low-OBP/high-SLG guy and the other a high-OBP/low-SLG guy — you’re actually better off with the high SLG guy.

If you have a great offense, you want the high-OBP, which the table shows in the rows pertaining to the 1999 Indians. Both Trumbo and Callaspo would make that offense worse, but Trumbo’s proficiency at making outs would do more harm to a team that was already stacked with sluggers. Teams that already get on base a lot can get a non-linear return from adding more OBP to the mix, so the slap-hitters are more valuable in that context, but you won’t get as much production from a guy who singles-and-walks his way on base in a line-up that is likely to fail at driving him him.

None of this is to say that Trumbo is actually what the Mariners needed, or a significant upgrade that will push them to the top of the AL West. He’s still a pretty mediocre player, especially when tasked with playing the outfield, and he’ll hurt the team’s defense almost as much as he helps the offense. Mark Trumbo’s low OBP is a legitimate weakness, and keeps him from being as good as people who just value HRs and RBIs think he might be.

But neither does his sub-.300 OBP make him worthless, and contrary to what might seem logical on the surface, the Mariners would not have benefited more by adding a comparable OBP-centric hitter instead of another all-or-nothing slugger. Sure, they would have definitely been better off had the D’Backs given them A.J. Pollock instead, but that’s because A.J. Pollock is just a far superior player to Mark Trumbo. The Mariners paid a marginal price to get a marginal player, and as Jeff said in his write-up of the trade, this isn’t actually all that significant of a move.

Just don’t get caught in the trap of thinking that Trumbo is somehow less valuable to Seattle than he would be elsewhere because he adds another low-OBP bat to a low-OBP line-up. That’s not how offense actually works.