According to research from Pizza Cutter, pitchers’ strikeout percentage per plate appearance (K/PA) stabilizes after they have faced 150 total batters. It just so happens that all five starting pitchers in the Cardinals’ rotation have recently surpassed this threshold, so I thought it would be fun to create a visual that pits their 2012 K/PA against career rates. The graph is below followed by some brief commentary (after the jump).
Being that we aren’t quite out of April, it’s a little early to read into most statistics but certain numbers become meaningful before others. Under the definition of ‘sample size’ in FanGraphs’ glossary, you’ll find a list of stats and the corresponding sample sizes needed before they achieve reliability. For offensive players, the first of those numbers is swing percentage, or how often a given hitter decides to swing the bat.
Theoretically, swinging less often is viewed positively since it suggests that the hitter could be cultivating a more disciplined approach. Selectivity is important for two reasons: (1) Hitters have a better chance to reach base via the walk if they resist swinging at pitches outside the strike zone, and (2) abstaining from pitches that would likely induce weak contact allows them to avoid making easy outs.
According to FanGraphs, swing percentage stabilizes after 50 plate appearances. Most of the Cardinals’ regulars have accumulated enough plate appearances for us to visit this stat and observe whether any obvious trends have emerged. It’s important to note that even though a trend is established after the stabilizing threshold (in this case, 50 PA) occurs, it does not mean that said player will continue to perform at the new rate, just that we can expect his performance to trend in that direction in the future.
The chart below portrays those Cardinals who have stepped to the plate at least 50 times this season and it pits their 2012 swing percentages (blue line) against their career swing percentages (red line). For the record, I’m using the PITCHf/x plate discipline numbers available at FanGraphs (as opposed to BIS data) for the reasons outlined by Colin Wyers in this Baseball Prospectus article. WARNING: I augment each individual player description with other statistics (strike out rate, line drive rate, walk rate, etc.) that have not yet stabilized, so while they are adequate descriptions of what has transpired thus far, they do not imply trends… yet.