# Does run differential in baseball indicate if a team is just lucky?

Is it a good assumption that baseball teams with good W-L records but not so good run differential are just lucky rather than good? I’m thinking in particular of this year’s Seattle Mariners. They are currently sitting 22 games over .500 and only 3 1/2 games behind the Houston Astros. Looking at the run differential, however, shows the Mariners basically even in the runs scored vs. allowed, while the Astros are + 183. Does this mean the W-L records are deceptive and that the Astros are actually a much better team and that the Mariners have just been really lucky so far this year?

The chances are pretty good they are lucky and will have a major cold streak later. Sometimes a team will have a good year despite a poor run differential. The 1984 Mets were 90-72 despite giving up 24 more runs than scoring. Sometimes luck, sometimes a good bullpen, sometimes a good manager (Earl Weaver’s Orioles teams were good for usually winning more games than the runs scored vs allow equations predicted).

But number of runs can be the result of luck, too. When Mighty Casey comes up to bat, it’s up to him whether he hits a home run or not. But if he does hit a homer, it’s not up to him whether he scores 1 or 4 off of it.

IIRC, the single-best stat for how good a team really is is on-base percentage.

Not a baseball guy, but I’m quite sure that I’ve read Nate Silver or some other sabermetrics guy mention that run differential is much more predictive of future success than W-L record. Not sure how on-base percentage compares.

Basically, yes. There’s a formula called the Pythagorean Expectation, which calculates how many wins a team should have based on their runs scored and allowed. It’s better for predicting future results than just looking at wins and losses.

The meaure of an offense is run scored. On base percentage is simply a component of that; power has a lot to do with it, too.

A team can get lucky in terms of the number of runs it scores, but over the course of 80-100 games, which where we now are in 2018, that evens out pretty quickly.

Run differential is an excellent prediction, at least in the short term, of team performance. You’d be well advised to bet on teams that have been unlucky in this regard to improve, and vice versa. It doesn’t always work out but it’s where the smart money is. That said, sample size matters. If a team in April is 7-9 with a positive run differential, that means very little.

The Mariners have been pretty resilient in close games, and lead the MLB in number of one-run wins (at one point about half their wins were by a single run). There were also games where they were clearly losing and so stopped trying, saving the best of their bullpen, etc. if you combine a lot of close wins with blowout losses you’re going to have a bad run differential (and as if today that differential is at -2 even though they have the 4th best record in all of baseball, and better than every NL team).

On the other hand I wouldn’t be surprised by a late season collapse. The Mariners have been letting fans down like that for decades. There’s a hope that this year is different but we’ll see.

A team that wins many close games will, of course, always be perceived as being tough, or resilient, stuff like that. It’s natural to see what a team is doing and project upon that elements of character, determination, etc. (or lack of it.)

Honestly, though, it’s just luck. The Mariners have just gotten lucky. They aren’t really a .600 team, they’re more like a .500 team. They’re not going to collapse because of some organizational curse or failure of character; they’re going to sliude back simply because they are not really as talented as their current W-L record suggests.

Of course, .500 may be enough. If the Mariners play just .500 ball the rest of the way they’ll be 90-72 or 91-71, which may get them into the Wild Card game. They probably won’t finish ahead of New York (or Boston, if NY overtakes them) but they have a decent chance of remaining ahead of Oakland, and no one else is really close. Tamp Bay and Anaheim are really far behind, though I guess anything can happen.

Of course, they have some glaring holes that could be plugged with trades. A good hitter to replace Ryon Healy would do them a few wins’ worth of good. The catching situation could use some help. King Felix could be a big factor if he comes off the DL healthy and up to his old tricks.

One thing not mentioned… Edwin Diaz. He’s the best closer in baseball this year. He broke the Mariners record for number of saves halfway through the season. He’s a big reason why they won those one run games, it’s not purely luck. Of course relying so much on one pitcher isn’t something you want to have to do.