The Stats MLBTR Readers Need To Know

Baseball is a numbers game and we use lots of stats here at MLBTR. Some of them are easy to understand (Prince Fielder hit 38 homers in 2011) and some of them aren't as simple (he posted a -5.2 UZR/150 last year). So here's a guide to some stats you see here and elsewhere. It's not meant to be comprehensive; there are lots more useful stats than the ones that appear below, but these are some important ones:

• OBP - On-base percentage shows you the percentage of time a player reaches base. The league average now hovers around .325. The NL got on base at a .319 clip this past season and American Leaguers reached at a .322 clip. To compute OBP, add hits, walks, and hit-by-pitches as the times on base, and divide this total by the sum of the player's at-bats, walks, hit-by-pitches, and sacrifice flies.
• SLG - Slugging percentage measures a player's extra base power. To calculate SLG, divide a player's total bases by his at-bats. Power hitters like Fielder and Albert Pujols regularly slug over .500, but league averages were approximately .400 this past season.
• You'll often see us list a player's batting average, on-base percentage and slugging percentage (always in that order) like this: .280/.340/.450.
• UZR/150 - The defensive metric Ultimate Zone Rating estimates the runs a defender saves or costs his team. UZR/150 shows a player's impact per 150 games played. Check out this two-part explanation for more detail and keep in mind that it's best to look at multiple seasons when evaluating a player's defense with UZR/150.
• K/9 - The number of batters a pitcher strikes out per nine innings pitched. Pitchers struck out 7.1 batters per nine innings in 2011.
• BB/9 - The number of batters a pitcher walks per nine innings pitched. Pitchers walked 3.1 batters per nine innings in 2011.
• HR/9 - The number of home runs a pitcher allows per nine innings. Pitchers allowed 0.94 homers per nine innings in 2011.
• GB % - The percentage of batted balls that are ground balls. The Cardinals led MLB with a 47.7% ground ball rate in 2011, while the Diamondbacks had the lowest ground ball rate in the league: 41.9%.
• SIERA - Skill-Interactive Earned Run Average estimates ERA through walk rate, strikeout rate and ground ball rate, eliminating the effects of park, defense and luck, according to Baseball Prospectus. It's one example of a defense independent pitching stat (DIPS).

Check out Baseball-Reference, FanGraphs and Baseball Prospectus for lots more stats and check out our transactions glossary and the one at Cot's for explanations of transactions terms. MLBTR first published a version of this post on April 2nd, 2010.