Interpreting PITCHf/x Data

The PITCHf/x data on FanGraphs is rather intuitive. PITCHf/x Type explains the frequency each pitch is thrown along with the total count of pitches. The PITCHf/x velocity shows how hard each pitch was thrown on average. The Horizontal Movement section shows the type of lateral movement each pitch gains on average. It is important to note that movement that is positive moves away from right-handed batters while a negative number moves in towards right-handed batters. The vertical movement explains the type of average vertical movement of each pitch.

The real data that FanGraphs possesses that other websites lack is the ability to easily compare frequency, velocity, and movement among different pitchers. This gives us the ability to come up with the Best Pitches of 2011 series that we recently released, and gives fans or baseball analysts the ability to easily compare pitches. This brings me to linear weights, which is one of the most original pieces of analysis in regards to PITCHf/x available online.

The Pitch Type Linear Weights (“Pitch Values”) section on FanGraphs attempts to answer the question, “Which pitch is a pitcher’s best weapon?”  The changes in run expectancy between an 0-0 count and a 0-1 or 1-0 count are obviously very small, but when added up over the course of the season, you can get an idea of which pitch typically yields the best results for a pitcher. If one pitch is hit especially hard or a pitcher can’t locate one pitch for a strike, these problems will show up using Pitch Type Linear Weights. Also, if a pitcher gets lots of strikes and outs with a specific pitch, this success will show up.

You’ll notice that there are two different types of Pitch Type Linear Weights: total runs by pitch (which is shown as wFB, wSL, wCB, etc.) and standardized runs by pitch (shown as wFB/C, wSL/C, wCB/C, etc.). The first category is the total runs that a pitcher has saved using that pitch. However, it is tough to compare these total numbers since pitchers throw different amounts of each pitch. The second category corrects for this, standardizing the values on a “per 100 pitch” basis. In other words, when you see wFB/C, that represents the amount of runs that pitcher saved with their fastball over the course of 100 fastballs thrown.

Context:

A score of zero is average, with negative scores being below average and positive scores being above average.

In general, pitches will generally fall somewhere between +20 and -20 runs, with the most extreme pitches touching +/-30. On a per 100 pitch basis, the range shrinks to around +1.5 to -1.5 runs. Again, you’ll see some extreme scores on either end of the spectrum, but that’s the range that most pitches and pitchers fall into.

Things to Remember:

● This statistic has limited predictive power. It can show you what pitches a pitcher has had success with in the past, but you should be careful in extrapolating those results and projecting the future. It’s a descriptive statistic, not a predictive one.

● Beware of sample sizes! Sometimes a pitcher may have a handful of his pitches misclassified, showing that he throws a slider when he really doesn’t. Also, if these handful of pitches are really successful or really unsuccessful, that pitcher could show up towards the top or bottom of the wSL/C leaderboard. In other words, before drawing conclusions from the standardized leaderboard, be sure that all the pitchers up there have thrown a large amount of the pitch you’re looking at.

● Pitch Type Linear Weights can also be used to evaluate hitters, seeing which pitches they have had most success against in the past. In this case, the values are flipped; positive values are a good result for the hitter, not the pitcher.

Links for Further Reading:

Pitch Type Linear Weights – FanGraphs

Pitch Type Linear Weights Explained – FanGraphs

Z-Scores and Pitch IQ Scores – The Hardball Times