Tom Tango wrote about starter and reliever batting average on balls in play (BABIP) on April 30th, and asked me if I could graph the relationship over time. I have enough data to back to 1974. Note once again that I use (Hits-HR)/(AB -(HR+K)) as my definition of BABIP. The difference in the graph is Starter BABIP – Relief BABIP (click graph for a larger version):

Starter and Relief BABIP, 1974-2011. Difference = Starter-Relief

I included a trend line for the difference to show that there was a recent shift between starters and relievers. For most of the history shown, starters induced a lower BABIP than relievers. That changed this decade, and now the relievers are ahead of the starters. All this relief specialization and role assignment may be paying off.