Introduction: Some basic baseball vocabulary to start off: At Bat (AB) – Each time a batter comes to the plate EXCEPT: walks, being hit by the pitch, and sacrifices do not count.
Plate Appearance (PA) – every time a batter comes to the plate – no exceptions
Hit (H) – Any time a batter hits the ball into play and reaches base safely Example: Dustin Pedrois hits a ground ball between the 1st and 2nd basemen. He reaches first safely. That is a hit. OR David Ortiz hits a ball over the Green Monster. He gets a hit.
Total Bases (TB) – counts each base a better reaches by hitting the ball: singles count 1, doubles count 2, triples 3, and home runs 4. Walks do not count. Example: Adrian Beltre bats 4 times. He gets a single, a walk and a home run. He has 5 total bases.
Batting Average (AVG) – the ratio of hits per at bat: Example: Jason Varitek comes to the plate four times. He makes 3 outs and gets one hit. He has a .250 AVG. (1/4) Example 2: Victor Martinez comes to the plate four times. He gets a single, a home run, a walk, and one out. He has a .667 batting average. (2/3 – remember walks do not count).
Slugging Average or Slugging Percentage (SLG): The number of total bases a batter gets per each at bat. Example: Albert Pujols bats 10 times and gets a 3 singles, one double, a home run and a walk. He makes 5 outs. His SLG is 1.000. (9 total bases divided by 9 at bats. Remember the walk does not count)
On-Base Percentage (OBP): The number of times a batter reaches base successfully (including by walks) for each Plate Appearance (not AB). Example: Manny Ramirez comes to the plate 4 times. He walks twice and strikes out twice. His OBP is .500 (2/4). Note that his AVG is .000 (0/2)
Inning Pitched (IP): The number of outs a pitcher records, divided by three. Example: Josh Beckett starts the game for the Red Sox and is removed from the game after the 6th inning. He has 6 IP. Example 2: Tim Wakefield starts the game for the Sox and is removed with one out in the 5th inning. He gets 5 & 1/3 IP.)
Earned Run: number of runs a pitcher allowed that did not occur as a result of errors or passed balls Example: In his last start, John Lester gives up a home run in the first inning. In the third inning, Derek Jeter hits a triple. On the next pitch, Jason Varitek lets the ball go off his glove and Jeter scores. Lester has one Earned Run and one Unearned Run. Another Unearned Run example: Cano hits a ground ball back to third, but Mike Lowell throws the ball into the stands. Then Posada hits a home run. The pitcher is only charged with one earned run because Cano reached base on an error.
Earned Run Average (ERA): The number of earned runs a pitcher gives up each 9 innings. Example: Beckett pitches seven innings and gives up 2 earned runs. His ERA is: 2.54 (2/7 x 9)
Sample size: The number of examples of data we have available to us. Example: during the first week of the season, the Red Sox play 5 games. Dustin Pedroia comes to the plate 20 times. Our sample size is 20 plate appearances – a small sample. Example 2: By the end of last season, Dustin Pedroia had come to the plate 714 times. Our sample size is 714 plate appearances – a much larger sample size.
Sample size is very important when looking at a player’s statistics. After the first week of the season, there will be many very weak hitters batting over .500 and many very good hitters batting .200. This is because they have not played enough to give us a large enough sample of their work to make a smart decision about them.
If we forget about the problem of sample size, we can jump to the wrong conclusions when we watch baseball: • The announcer tells us that Mike Cameron has hit .333 against Mariano Rivera in his career. We assume that means that means he is good against Rivera, but if Cameron has one hit in three at bats, we really don’t know anything about his skills against Rivera. • The announcer tells us that Jason Bay is a great clutch hitter because he batted .400 in the playoffs, but if Jason Bay only batted 15 times in the playoffs, we don’t really know anything about his clutch ability.