Across different eras of baseball research, most analysis has been done on either balls in play or balls and strikes. But what can foul balls tell us?

Do changes in leading indicators have the same effects on pitching performance regardless of their starting point?

Leading indicators are useful for predicting changes in player performance. Do changes in these indicators have the same effects on performance regardless of their starting point?

Does looking at September+April splits better predict changes in performance over the rest of the season than looking at April splits alone?