On-base plus slugging percentage may seem like a great stat to use when determining a player's value, but it can be a tad bit misleading. Sometimes players play in ballparks in lighter air, giving the ball a better chance to go over the fence (see: Coors Field). Sometimes players play in ballparks with nearby ocean air, which forces the ball to not go as far (see: Petco Park). Every ballpark has different dimensions and different weather conditions. This can cause a player's OPS to fluctuate on an increase or decrease, based on where they play.
Don't worry, however, there is a solution. A stat called OPS+ is the solution, an OPS statistic adjusted to that player's ballpark and which league the player played in (American League or National League). The formula for OPS+ is:
OPS+ = 100 x (OBP/lgOBP*+SLG/lgSLG* - 1)
The easy way to understand lgOBP* and lgSLG* is that they are park-adjusted league-average on-base percentage and slugging-percentage. Basically, it's the average of how well the players played in those leagues in their ballparks. ESPN's Park Factor shows this equation for adjusted runs in a specific ballpark, thus creating lgOBP* and lgSLG*:
((home runs scored + home runs allowed)/(home games)) / ((road runs scored + road runs allowed)/(road games)
An OPS+ of 100 is exactly average, making it even easier to determine the value of a player. An OPS+ of 150 or greater is excellent, 125 is good, while an OPS+ of 75 is poor. Using this stat, you can determine how much better a player is than average (100 OPS+) or how much better a player was compared to another player of another era.
For example, if player A had an OPS+ of 110, and player B had an OPS+ of 80, player A can be considered to drive in 30% more runs than player B.
The only problems in OPS+ is that the stat weighs on-base percentage and slugging percentage equal to one another. Sabermetricians agree that OBP is more valuable than SLG, which somewhat throws the stat out the window. OPS+ is completely an offensive stat as well, and does not factor in other ways a player can be valued, in base running and fielding. Players who are valued higher because of defense can sometimes be given a misleading OPS+ stat.
Here are two quotes from a Yahoo article about OPS+:
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 10 OPS+ players in 2013 were: Miguel Cabrera (181), Mike Trout (179), Chris Davis (165), David Ortiz (160), Paul Goldschmidt (160), Andrew McCutchen (158), Jayson Werth (154), Joey Votto (154), Josh Donaldson (148), and Edwin Encarnacion (145). While the top 6 were "no-brainers," the rest of the list goes to show that these players are far more underrated than people think.
OPS+ is a great statistic that proves a player's worth in one number. Easy as pie.
Pitching has a similar stat. It is called ERA+.