An open-source framework for analyzing MLB player projection accuracy
Whiffs does the normal MAE and RMSE analysis you might find in other projection evaluations, then takes it to the next level. It scales projections to a consistent league average and weights errors by playing time. (Check out the methodology for details.)
It currently has accuracy data for Marcel, Steamer, ZiPS, Razzball/Steamer, and Davenport projections going back as far as 2010. (Got more? Email me at mayscopeland@gmail.com.)
Playing Time Projection Accuracy
Plate Appearances (PA)
Batters Faced (BF)
A note on playing time
ZiPS doesn't really forecast playing time—it's showing how players might perform if given a full season of MLB playing time. Their "what if" scenarios are heavily penalized by the Whiffs methodology, which includes players with even a single major league PA or BF.