For every run scored, a run is allowed. That’s cemented in the rules of the game and the laws of physics. To score a run, someone else has to let a run score. While most of that allowance belongs to the pitcher, there are eight other players on the field with him at any given moment and they deserve some of the blame or credit for how often a run does or doesn’t score.
Unfortunately, for most of baseball history, we’ve had exceptionally lousy practices for measuring defense. Errors and fielding percentage seem to make sense at first but if you peel back the onion at all, they just don’t get the job done. Defense is a meaningful slice of run prevention and we care about measuring run prevention, ergo, we need to do a decent job of measuring defense. How do we go about doing that?
This section of the Library walks you through that process, chronicling some of the different tools we have at our disposal. There are going to be some changes happening in the next few months and refines along the way, so think of this as a bit of a living organism.
Some additional, incomplete links (suggest others!):