MLB will see 10 rookie umps replace veterans as position faces more changes, challenges than ever - The Athletic
MLB umpire rankings: Pat Hoberg is baseball's most accurate ump
Observations of Umpires at Work – Society for American Baseball Research
MLB umpires are good at calling balls and strikes, but they need help - The Washington Post
Umpire (baseball) - Wikipedia
MLB: Umpire Hernández blew calls, losing World Series job
Two wrongs trying to make a right – makeup calls are common for MLB umpires, financial analysts and probably you
Umpire (baseball) - Wikipedia
The Umpire Strikes Back - WSJ
Should MLB use an automated strike zone in place of umps? “Yes, a thousand percent yes.” – The Denver Post
Why is MLB still using 6 umpires in postseason games? - Bleed Cubbie Blue