Is there an adjustment period for players when they change leagues?

If Alex Rodriguez moves to a National League team, is he going to have an adjustment period? Do players in general need some time to adapt when they move from one league to the other? The leagues are different. One has the DH and the other doesn't. Over the last few years the AL has had the more dominant teams and has outperformed the NL in interleague contests.

Here's how we look at this question. This past season there were 23 full-time players who switched leagues. They were full-time players in each of the 2006 and 2007 seasons. We compared their performance in each league to see if their performance improved or declined. We used Win Shares to help us evaluate overall performance. If there is an adjustment period, you would expect players to decline more often than not when moving from one league to the other.

The result: no adjustment period. Or so it appears when comparing these players from 2006 to 2007. Most of them had similar performances in the two leagues—13 guys. These 13 players had win share totals within two win shares comparing 2006 to 2007. There were 10 players who had a win share difference greater than two between the two seasons. Those 10 guys split it down the middle. There were the same number of guys who improved as the number who declined—five. Here are the details.

Players with improved performance