Since June 6:
After both starting the year pretty well, the Duke and Rivera have headed in totally opposite directions. Duchscherer's post-June 6th WHIP is double his pre-6/6 WHIP and his K/BB has been halved. Rivera, meanwhile, has decreased his WHIP to an impressive 0.91 and more than tripled his K/BB rate. Simply put: Rivera is much better.
(1) Don't get too excited about small sample sizes.
(2) Don't point out how well A's pitchers are doing and invoke the "Jinx."
(Read this for an explanation of why lesson #2 is a joke.)