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Velocity and K/9

One of the things that I’ve been wanting to look into this off-season is the relationship between velocity and effectiveness. As we all know, major league pitchers are selected off of the strength of their fastball more than anything else. Body type, breaking balls, performance – all of those fail to receive the same level of confidence as fastball velocity. If a guy can throw in the upper 90s consistently, he’s going to get a chance to work out all his other issues. If a guy can’t break 80, he’s going to have to be phenomenal at everything else to even get a crack at a major league job.

However, we know that velocity isn’t everything. Command, movement, the ability to mix pitches – these all count, and in many cases, they count a lot. Jamie Moyer is the obvious example that everyone points to. It’s clear that velocity isn’t a prerequisite for major league success, but that doesn’t really give us an answer for how important it is.

To start looking at the issue, I’ve taken the 427 pitchers who accumulated at least 30 IP in the majors last year and plotted their velocity and K/9 rate on a graph. Rather than talk about it some more, I’ll just show you that graph.


You can click on the graph to see the full version, by the way.

First off, there’s an obvious relationship. The regression line through the middle trends up, which fits with common sense – guys who throw hard strike out more batters than guys who don’t. But perhaps the slope of the line isn’t quite what you might have expected? It’s lower than I thought it would be, honestly. While there are guys like Jonathan Broxton and Fernando Rodney who fit right into the high velocity/high strikeout rate category, there’s also Brandon League and Matt Lindstrom – the two hardest throwers in the sample, and they posted K/9 rates of 6.27 and 6.75 respectively.

If you look down in the right hand corner, you’ll notice the r squared, which is the coefficient of determination. This number, .2299, essentially means that if you had a pitcher’s single year velocity data, you’d know about 23% of what is necessary to know his strikeout rate for that year. The other 77% of strikeout rate is not explained by how hard he throws his fastball. Now, since these are just single year samples, a portion of that unexplained K/9 rate will be noise, so don’t take that to mean that 77% of strikeout rate is command, off-speed stuff, and other factors all under the pitcher’s control. There’s variables outside of what the pitcher can influence that are in play, too – the umpires, the opposing batters, etc…

Still, though, it’s important to know that if you’re trying to predict strikeout rate, velocity is about 1/4 of what you need to account for. That makes it likely to be the biggest factor, but it’s not so dominating as to exclude the other things besides throwing hard. A high velocity fastball is a good thing, but it is definitely not the only thing.