Big League Stew - MLB

The world of advanced baseball statistics can be an intimidating place for those of us who slept our way through advanced algebra or haven't been a follower of the Bill James revolution from the beginning. 

Still, that doesn't mean that we should feel left out when it comes to another way of understanding and appreciating the game we all love. With that in mind, BLS stat doctor Alex Remington will explore a new advanced statistic each week during the offseason, providing a simple primer for the uninitiated.

Today's statistic: OPS+

What it stands for: Adjusted On-base percentage Plus Slugging, as calculated by Sean Forman of baseball-reference.com

How they calculate OPS+: The basic principle is to take a player's OPS (which is the sum of on-base percentage and slugging percentage), adjust it for different ballpark factors and then put it on a percentage scale. When it comes to OPS+, 100 is the league average, 110 is 10 percent above league average, and 90 is 10 percent below league-average. The actual number of ballpark variables involved makes the calculation complicated to write out here, but the actual arithmetic is simple.

OPS+ = 100 x (OBP/lgOBP*+SLG/lgSLG*- 1)

I know what your'e asking: What are lgOBP* and lgSLG*? They're park-adjusted measures for the league-average OBP. Park factors can be very complex and are generally beyond the scope of this article, but ESPN's park factor formula for runs is relatively simple and gets at the basic concept:

((Runs scored at home + Runs allowed at home)/(Home games))/((Runs scored on the road + Runs allowed on the road)/(Road games))

What OPS+ is good for: If you have access to baseball-reference.com, which automatically calculates OPS+, it gets rid of your need for OPS. OPS+ does share the core weakness of OPS — namely, it gives equal value to both OBP and SLG, though sabermetricians agree that, point by point, OBP is a more valuable stat than slugging.

But because OPS+ incorporates league average and park factors into the calculation, it makes comparing one player to another much easier, both for a single year and across leagues and different eras. For example, Mickey Mantle and Albert Pujols(notes) have the same career OPS+ (172) while Babe Ruth is the career leader with a whopping 207 mark.

How OPS+ works: OPS+ is a measure of offensive productivity. It is neither the best nor the easiest to calculate, but it occupies a middle ground between metrics that are overly simple, like OPS, and metrics that are more conceptually complex, like wOBA and WAR.

OPS+ is really easy to express: 170 is really good, because anyone who's 70 percent better than the league average is clearly a star. It's on the same scale as its counterpart, ERA+, which is a version of ERA that's similarly normalized for park factors and league average and put on a 100-point scale.

Conceptually, it's easy to understand: it's a single number that attempts to measure a player's contributions at bat. It doesn't account for baserunning or defense and it doesn't account for the discrepancy in value between on-base percentage and slugging, but it's a pretty good indicator of how good a year a particular player has had.

The top performers in OPS+ are the top hitters in the league (in 2009, it was MVPs Albert Pujols, 188, and Joe Mauer(notes), 177) the players around 100 are all league-average hitters, and the worst performers in OPS+ are all the worst (the wretched Emilio Bonifacio(notes), 63, and Yuniesky Betancourt(notes), 66).

When OPS+ doesn't work: Because it doesn't account for the relatively higher value of OBP, not all OPS+ scores are created equal. For example, Yadier Molina(notes) and David Ortiz(notes) both had a league-average OPS+ of 100 this year, but they were clearly not equal.

For one thing, Molina is a good defensive catcher at one of the most important defensive positions on the diamond and Ortiz doesn't play defense at all.

For another, Molina had an OBP of .366 while Ortiz had an OBP of .332. So while Papi slammed 28 homers while Molina only poked seven, Yadier was still a significantly more valuable player in 2009 because he made less outs and reached base more frequently. There are more advanced metrics that take more of these distinctions into account, but buyer beware: The more moving parts in any model, the more room for something screwy to happen to the result.

Why we care about OPS+: Unlike BABIP, OPS+ isn't a "necessary" metric. It takes a lot of things into account — league-average performance and the relative effect that individual ballparks have on offense — but fails to take a lot of other things into account. Because it's so complicated to calculate, basically requiring a dedicated database with a constant pipeline of data, I only recommend using it when someone else does the heavy lifting of calculating it.

But it's really, really easy to use, and the percentage scale is really easy to understand. By its very nature, it's designed for the middle of the road. Many diehard baseball fans still don't really understand advanced stats like wOBA, WAR, or the much-maligned VORP, but they're often familiar with OPS+. It's easy to explain, because it's just a much-improved version of a stat we've all been using for years. Think of it as training wheels for the more advanced stuff over at Fangraphs and Baseball Prospectus.

Next week's lesson: FIP

Previous lessons: BABIP

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