
Umpires can have an effect on MLB games. Each one has his own strike zone thus there will be trends regarding walks and strikeouts. Just like looking into any trends, it is important to weigh it accordingly. I have seen plays solely based on Umpire trends which is ridiculous. No more so than basing a play/wager on any other trend. We do want to ride streaks such as winning and losing streaks, or hot scoring offenses or cold ones, so we must be good at identifying trends and streaks.
Gamblers look to find any edge they can get. It should be no surprise that umpires have gimbling stats! Their stats include Home Team wins and losses, average total, walks, strikeouts, and more depending on where you get the data from. The chart below is a sample form last year.

The premise of ANY trend should be to validate or further solidify reasoning that supports a position in the game. For example, let’s say you like Over 9.5 runs in a game. You find that Cory Blaser is the Umpire. He has an avg. total runs scored 10.36 in his games. This trend is supporting your Over premise. We cannot just take this information and play over based on the total trend for the umpire. Keep in mind, these trends do not show what games he was part of. We do not know if he was in hitter-friendly stadiums or if he was part of some kind of crazy scoring games that skew the data. We can look to see if there is justification for runs being scored due to his strike zone. This is the important part.
A small strike zone means less balls in the strike zone and more balls out of it therefore there should be more walks. The more guys on base creates more scoring opportunities hence there should be a higher total. This works opposite as well. The larger a strike zone is the more strikes thrown thus more strikeouts. This creates less guys on base and lowers the scoring opportunities. These are logical reasonings for the total average which in turn creates legitimacy to repeatable future strike zones, thus consistency in totals markets for this umpire.
A couple of thoughts. One, do not ever make any wager solely based on a trend no matter what that trend is. Two, a trend only has value if there is some reasoning to it which can make it repeatable for a reason. For example, Team A might have won 6 Weds day games in a row. Is that a repeatable/reliable trend? Maybe? Look deeper. Wednesday are get away days. It means teams tend to play early day games so they can fly out to their next city that night and play a game on Thursday night. An origination or manager may be very good at motivating his team in these games. They might be specific “go after” to win type games where the organization has identified to attack these spots to win more games. If you can find reasoning like this, then yes these are repeatable and reliable trends. However, more often than not, you will not be able to support the trend with legitimate analysis. Three, make sure you know if you are betting a streak or a trend. Baseball is a game of streaks over the course of 162 games. We need to ride streaks. Do NOT blindly ride trends!