For years, Baseball's best home run hitters have participated in the annual Home Run Derby. However, the annual derby has often caused hitters to slump. To judge the substance of the "Post-Derby Slump" myth, I looked back at a couple of home run derbies from the past decade and compared the stats in the Society for American Baseball Research. 

According to the SABR, “A variety of sources have indicated the existence of a Home Run Derby curse." For example, Alex Rodriguez, fourth all-time in home runs, spoke about the Home Run Derby during his tenure with the Yankees, stating, “I try to stay away from that” and “My responsibility is to the New York Yankees. I need my swing to be at its best.” To Rodriguez, the implication is that participation in the derby would leave his swing somewhere other than at its best. In the Wall Street Journal, we read that “for each of the past four years, one player who has hit at least 10 home runs in the Derby has seen his power disappear once play resumed for the second half of the season.”

According to MLB.com, five players did worse after the derby compared to three doing better. The players were determined by looking at their OPS (On-Base Plus Slugging) before and after the derby. 

The players who got worse: 

-.200 points // Aaron Judge, 1.139 to .939

-.164 points // Miguel Sanó, .906 to .742

-.074 points // Justin Bour, .923 to .849

-.065 points // Mike Moustakas, .863 to .798

-.060 points // Cody Bellinger, .961 to .901

The players who got better:

+.162 points // Giancarlo Stanton, .933 to 1.095

+.114 points // Charlie Blackmon, .950 to 1.064

+.046 points // Gary Sánchez, .850 to .896

This is not a one of either according to SABR who studied every derby participant from its start to 2010 saw a decrease in OPS in the Second Half of the season when all the player’s numbers were averaged together. 

So, after looking at the data pre-derby and post-derby, I have concluded that a post-derby slump exists.