Baseball is a numbers game and we use lots of stats here at MLBTR. Some of them are easy to understand (Albert Pujols hit 47 homers last year) and some of them aren't as simple (Pujols posted a 0.8 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 usually hovers around .330 and last year was no exception (.331 in the NL and .336 in the AL). OBP is computed by adding hits, walks, and hit-by-pitches as the times on base, and dividing that 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. SLG is calculated by dividing a player's total bases by his at-bats. Power hitters like Adam Dunn and Ryan Howard regularly slug over .500, but league averages are usually closer to .420.
- 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 - Ultimate Zone Rating is a defensive metric that 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. Last year pitchers struck out 7.0 batters per nine innings.
- BB/9 - The number of batters a pitcher walks per nine innings pitched. Last year pitchers walked 3.5 batters per nine innings.