In my series’ first part, I looked at the effect that Park Factors have on various ERA estimators. The original question I attempted to answer was whether certain estimators were better suited for predicting performance, depending on whether a park is hitter-friendly or pitcher-friendly. The short answer was that ERA estimators did a much better job in hitter-friendly parks than pitcher-friendly parks, relative to YR1_ERA.
One question I didn’t answer was whether the effectiveness of estimators in various types of parks also varied by pitcher role (i.e. starters versus relievers). Generally speaking, ERA estimators perform better when you restrict the analysis to starters only — since relievers tend to be more volatile year-over-year. The question is whether this same pattern will hold given park factors’ impact. And as predicted, ERA estimators do a better job predicting performance for starters versus relievers.
The current data set includes 533 pairs of starter seasons and reliever seasons where the pitchers threw in the same parks in the first and second years, and did so as starters or relievers both years. Before segmenting by park type, we see results that are consistent with previous analysis regarding ERA estimators and their predictive powers for starters and relievers: