Specifically, to what extent to do park factors affect the usefulness of various ERA estimators? It seems reasonable to assume that while much of what happens when a ball is put in play is not controlled by a pitcher.

However, given that some extreme parks are likely to exercise their own environmental force over the outcome of batted balls it stands to reason that ERA estimators that factor in a pitcher’s batted ball profile may do a better job in certain types of parks than others.