Baseball statistics have gotten increasingly complicated over time. Once upon a time, people primarily used counting stats like RBIs, home runs and stolen bases. Some of the most complicated statistics were stats like batting average, fielding percentage and ERA basically requiring little more math than average division. Then, there were statistics like FIP, wOBA and WAR. These are difficult to calculate, but are reasonably simple to understand. A higher wOBA and WAR is better than a lower one. Now, with data from Statcast and Pitch Fx available to the public, it allows us the creation of new statistics that are difficult to both calculate and understand. One of these metrics is exit velocity.
Exit velocity is a relatively new stat that I believe was introduced with the creation of Statcast. It measures how the speed of a baseball after it is hit by a batter. According to Sports Illustrated, the Rays solely use exit velocity to measure batter performance. It’s also popular with players. Jake Lamb has said that he thinks that exit velocity tells you exactly what you need to know and that he likes this stat even though he generally doesn’t care for stats. It probably didn’t hurt that this stat ranked him highly.
However, it can be difficult to understand. Take ERA for example. There’s no special bonus for giving up a low amount of runs just like there’s no special penalty for giving up a large amount of runs. Each run allowed per nine runs increases a pitchers ERA by a run. Each double hit increases a batters OPS the same as every other double. In a word, these statistics are linear. You put all the numbers into a formula and it spits out an answer that values each result similarly.
It isn’t clear that this would necessarily be the case for exit velocity. If one hits a pitch lightly, then it means that a fielder may have to charge the grounder and make a throw to first while running to get the out. Depending the game situation, the fielder’s ability or the runners speed, a fielder may decide not to make the throw. If he does make the throw, there’s the possibility that the runner will beat the throw or that it will result in an error. In contrast, a grounder hit at a normal speed will likely be easily fielded with enough to throw for first for the out. When hit into the air, it will likely result in a routine fly ball. If exit velocity isn’t linear, then it means that we’ll need to come up with different ways to understand it.
My hypothesis is that graphing exit velocity by wOBA would result in a parabolic shape. Balls with high exit velocity would have the highest average wOBA. Those with the lowest exit velocity would have a medium wOBA, and those with average velocity would have the lowest wOBA. In order to test my hypothesis, I downloaded a summary table created by Baseball Savant that has exit velocity and wOBA and graphed my results. It turns out that I was somewhat right and wrong.
The chart below shows the average wOBA by exit velocity as measured in miles per hour. The graph isn’t a parabola per se, but there’s definitely some similarity. Batters do extremely well when they hit the ball over 100 mph. From 91-99 mph, their performance begins to decrease significantly. In the sample measured by the summary table, batters had a .542 wOBA when they had an exit velocity of 99 mph and a wOBA of .257 when they had an exit velocity of 91 mph. Batter wOBA continues to drop based on exit velocity until batters hit the ball at 35 mph. Once exit velocity drops to 35 mph, wOBA increases with it to the .350 mark. It seems that hitting the ball very lightly may be better than hitting the ball 90 mph. Here’s the chart.
This next chart shows average wOBA based on exit velocity as measured in groups of either 5 or 10 mph (only for 50-59 and 60-69 mph). This chart is probably a bit clearer to understand than the first one and clearly shows how a small increase in velocity leads to a large increase in production only if the ball is hit with an exit velocity of over 95 mph. It does seem to show that batters have a small bump in performance if they hit the ball between 60-74 mph, but this could just be a quirk of the data that wouldn’t show up if we used a different sample.
These charts show why exit velocity is different than other more standard stats. There’s a huge bonus for hitting the ball really hard. But if you hit the ball at an average speed, then it doesn’t really matter much if you hit the ball 60 or 70 mph. Your results will likely be substandard. There is also evidence that suggests that an exit velocity of 20-34 mph will result in better results than hitting the ball at 60 mph. To be fair, there were only 119 balls put into play between 20-34 mph and Statcast has difficulty measuring pitches hit that weakly. This may just be the result of an error in the data and may not actually exist in real life.
This is why there’s only a .561 correlation between exit velocity and wOBA when looking at all balls put into play measured by Statcast by all batters. It’s true that a hard hit ball is better in general than a softly hit ball, but that doesn’t make it true in all instances. As such, this is one reason why average exit velocity has limited value.
Instead of looking at average exit velocity, it is therefore important to look at exit velocity in bins. Pitches hit with an exit velocity of 105+ mph should be considered highly valuable. Pitches hit with an exit velocity between 100 and 105 should also be considered successful at bats. Pitches hit between 95-99 mph are not as valuable but still likely to be somewhat productive. Anything lower than 95 mph should probably be ranked as needing improvement.
Potentially, this may mean that a batter with an average exit velocity of 85 mph can be less valuable than a batter with an average exit velocity of 75 mph. If the batter with an average exit velocity of 85 generally puts the ball into play with an exit velocity between 80 and 90 mph, then he’s not going to be productive. However, a batter with an average exit velocity of 75 mph but puts a lot of pitches into play with an average exit velocity of over 100 mph or under 40 mph could be extremely useful. Especially if he’s a quick runner and can beat out a tough throw to first. In addition, it’s possible that an acceptable exit velocity depends on the type of pitch thrown.
Some of the new statistics available to the public are excellent and can help advance our understanding of baseball but don’t necessarily work like statistics have in the past. As a result, we’ll have to use different methodologies to gauge their value and what they’re telling us about baseball players.