Here I show how relief pitchers get significantly worse results when hitters see them on multiple occasions in a short time period.
*Note - There are selection biases in the graphs below that can cause pitcher overperformance in the initial matchup for several samples. I've added an appendix with graphs that exclude this selection bias to show that the effects are still real (albeit with a smaller magnitude than would be suggested by the original analysis). Find the appendix at the end of the original post.
The effects of familiarity on hitter-pitcher matchups in the postseason are often discussed on broadcasts and in articles, however I haven't seen much research on the subject. This led me to investigate the question myself with surprising results.
I took data from the 2010-2021 seasons, filtered out all the starting pitchers, and in cases where relievers pitched over one time through the order I only selected their first matchup with a particular batter in a game. This allowed me to group results by the number of times a particular hitter-pitcher matchup occurred to see how results change the second and third time that two players face up.