Hardball times did a study of umpires' ball and strike calls in 2015+2016, and Joyce was #1 in terms of calling it correctly within the typical ball/strike parameters. He scored 139 where 100 is average.
http://www.hardballtimes.com/mlb-umpires-2016-review-world-series-preview/It is important to understand what this metric is measuring. Note that I use the word expected. It is ascribing positive value to calls that agree with the league majority for a given pitch location and negative value otherwise. This does not necessarily mean the call is correct, based on the rule book strike zone, which means this does not necessarily identify the best home plate umpire. An umpire may have a unique strike zone he calls quite consistently, and there is certainly an argument to be made that being consistent in any one zone is fine. However, this metric rates umpires on how well their calls agree with what is being called around the league.
In my opinion, given that all home plate umpires are evaluated and provided with feedback after each game, over the course of an entire season the aggregate of all called pitches is a good proxy for the strike zone the league wishes to have called. I also believe there is value is understanding how much an umpire tends to differ from the league average as far as the calls he is making, in terms of how expected his calls are and how often his unexpected calls are strikes on pitches that are more commonly balls and vice versa.
I calculated these numbers for all umpires in both the 2015 and 2016 seasons. Taking the seventy umpires who worked the most behind home plate over both seasons, there was a correlation of 0.66 in the Expected+ scores between the seasons. This suggests calling balls and strikes per the typically called league zone has a sizable degree of skill that is repeatable between seasons for umpires.
