Defense (Def) is the combination of two important factors of defensive performance: value relative to positional average (fielding runs) and positional value relative to other positions (positional adjustment). In order to properly evaluate a player’s defensive value, you need to know both factors and adding them together and providing it on the site saves you a bit of legwork.
Def is a useful statistic to consider because saving a run in left field is easier than saving a run at second base because the average baseline of performance is much lower. If you were to just look at fielding runs, you would be underrating defenders at more difficult and important positions. There’s no reason why you can’t look at the two components separately, but we offer the option to simply use Def to make comparisons among different position players easier.
The equation for Def on FanGraphs is below:
Def = Fielding Runs Above Average + positional adjustment
Ultimate Zone Rating (UZR) is the defensive metric used in our WAR calculations to measuring fielding runs above average relative to the average player at that position. If you wanted to use Defensive Runs Saved (DRS) or any other related metric, you could do so manually, but our Def calculations use UZR for all years since 2003. Prior to that we use Total Zone (TZ) because UZR data does not exist prior to 2003 and TZ is the best available measure of fielding runs for earlier seasons.
The positional adjustment is used to account for the fact that defensive positions vary in their difficulty and importance. In other words, a league average shortstop prevents substantially more runs than a league average first baseman and we need to find a way to account for this when determining overall value. Adding the positional adjustment to the player’s fielding runs does exactly that. In the “Value” section of the player pages and leaderboards, you can find each player’s fielding runs and positional runs. Adding them together results in Def.
There is no reason why you should use Def instead of looking at fielding runs and the positional adjustment separately, but if you want a single measure of overall defensive value, our Def stat does the trick. The important thing to remember is that all of the leading defensive metrics (UZR, DRS, TZ, etc) are presented relative to the average defender at that position. This means that a +10 defender in left field is 10 runs better than the average fielder in left field. A +10 defender at shortstop is 10 runs better than the average shortstop. These are not equal because the average shortstop is a better defender.
To read more about our positional adjustments, head over to this page of the Library.
Def gives you a measure of the player’s total defensive value that you can use to compare to all players, rather than just players at his position. A -10 Def is the same value no matter the position, which makes it useful when looking at a grouping of players that play different defensive positions.
How to Use Def:
If you know how to measure players using run values, Def is very easy to pick up. Def measures the numbers of runs above or below average a player is worth when combining their fielding runs and positional runs. Zero is league average and every ten runs (approximately) in each direction is a win above or below average.
If you’re new to run value stats, the link above will be helpful, but the simple explanation is that run value stats combine the quality of a player’s performance with how much they played. If you played one inning of defense but made the best play of the year, are you the best defender? Using a rate stat, you might be, but when we want to talk about value, we need to scale that performance to the number of innings or games or chances you had. So fielding runs or Def simply gives credit for the how well you played and how much you played.
If you’re totally new to defensive metrics, click on one of the links to UZR, DRS, or TZ above for a primer on how they work. One of the important limitations of these stats is that they can be unreliable in small samples, which means that over the course of twenty or thirty games, Def won’t hold much predictive value either. A full season of data or multiple years of data will be more useful in determining the quality of a defensive player.
Def will always center around zero in each season, but there will be some variation. The following are basic rules of thumb. Use them as a guide:
Rules of Thumb
Things to Remember:
● Def is fielding runs above average and positional runs added together, so you can use it to compare players at different positions.
● Def is equally as volatile as the defensive metrics used to build it, so treat it just like you would treat UZR or DRS with respect to sample size.
● Def controls for park and run environment because UZR does, so you can compare it across season and park.
● League average is always set to zero.
● About 10 runs are equal to one win.
Links to Further Reading:
Neil Weinberg is the Site Educator at FanGraphs and can be found writing enthusiastically about the Detroit Tigers at New English D. Follow and interact with him on Twitter @NeilWeinberg44.