Stats Glossary

Weighted On-Base Average (wOBA) is based on a simple concept: not all hits are created equal. Batting average would have you believe they are, but think about it: what’s more valuable, a single or a homerun? Batting average doesn’t account for this difference and slugging percentage doesn’t do so accurately (is a double worth twice as much as a single? In short, no). OPS does a good job of combining all the different aspects of hitting (hitting for average, hitting for power, having plate discipline) into one metric, but it weighs slugging percentage the same as on-base percentage, while on-base percentage is more valuable than slugging.

Weighted On-Base Average combines all the different aspects of hitting into one metric, weighting each of them in proportion to their actual run value.

Weighted Runs Above Average (wRAA) is based off of wOBA, and measures the number of offensive runs a player contributes to their team. How much offensive value did Evan Longoria contribute to his team in 2009?  With wRAA it’s easy to answer that question: 28.3 runs. Zero is league-average, so a postitive wRAA value denotes above-average performance and a negative wRAA denotes below-average performance. This is also a counting statistic (like RBIs), so players accrue more (or less) runs the more they play.

Calculating wRAA is simple if you have a player’s wOBA value: subtract the league average wOBA from your player’s wOBA, divide by 1.15, and multiply that result by how many plate appearances the player received.

Weighted Runs Created (wRC) is an improved version of Bill James’ old Runs Created (RC) statistic, which attempted to quantify a player’s total offensive value and measure it by runs.  This way, instead of looking at a player’s line and listing out all the details (e.g. 23 2B, 15 HR, 55 BB, 110 K, 19 SB, 5 CS), you could synthesize all the information into one metric and say, “Player x was worth 24 runs to his team last year.”  While the idea was sound, James’ formula has since been superseded by Tom Tango’s wRC  and is based off of wOBA.

Similar to OPS+, Weighted Runs Created Plus (wRC+) measures how a player’s wRC compares with league average.  League average is 100 and every point above 100 is a percentage point above league average. For example, a 125 wRC+ means a player created 25% more runs than league average. Similarly, every point below 100 is a percentage point below league average, so a 80 wRC+ means a player created 20% fewer runs than league average.

wRC+ is also park and league adjusted, meaning you can use it to compare players that played in different years, parks, and leagues.  Want to know how Ted Williams compares with Albert Pujols in terms of offensive abilities?  This is your statistic.

Batting Average on Balls In Play (BABIP) measures how many of a batter’s balls in play go for hits. While typically around 30% of all balls in play fall for hits, there are three main variables that can affect BABIP rates for individual players:

a) Defense – Say a player cracks a hard line drive down the third base line. If an elite fielder is playing at third, they may make a play on it and throw the runner out. However, if there’s a dud over there with limited range, the ball could just as easily fly by for a hit. Players have no control over the defenses they’re facing, and they can only direct their hits to a limited extent. Sometimes a batter can be making good contact, but is simply hitting balls right at fielders. Also, a batter that consistently hits into a shift may have a lower BABIP than a typical player.

b) Luck – Sometimes, even against a great defense, bloop hits can fall in. A batter may turn a nasty pitch into a dribbler that just sneaks past the first baseman, or they may blast a shot in the gap that a fielder makes a diving catch on. Hits can fall in despite the best pitches and the best defenses – that’s just the game.

c) Changes in Talent Level – Over the course of a season, players can go through periods of adjustment. Maybe pitchers adjust to a weakness that a batter has, and the batter starts making less solid contact and getting fewer hits. Maybe a batter is simply on fire for a season, playing at a very high talent level and roping hard line drives all over the field. The harder a ball is hit, the more likely it is to fall in for a hit.

Due to this flakiness, BABIP can dramatically affect a hitter’s batting average. If a large number of a batter’s balls in play go for hits, that can boost his batting average quite high. Similarly, if a large number of balls in play get caught, it can reduce a player’s total offensive value.

If a player has a very high or very low BABIP, it means that whatever the reason for the spike (whether it’s defense, luck, or slight skill), that player will regress back to their career BABIP rates. BABIP rates are flaky and prone to vary wildly from year to year, so we should always take any extreme BABIP rates with a grain of salt.

Below is a list showing how many PAs it takes for a statistic to become reliable.

50 PA: Swing %

100 PA: Contact Rate

150 PA: Strikeout Rate, Line Drive Rate, Pitches/PA

200 PA: Walk Rate, Groundball Rate, GB/FB

250 PA: Flyball Rate

300 PA: Home Run Rate, HR/FB

500 PA: OBP, SLG, OPS, 1B Rate, Popup Rate

550 PA: ISO