How Is xBA Calculated?

[kahudson]

I checked the glossary and did not see the explanation and formula for calculating xBA. I'm trying to understand why the xBA for Ramon Hernandez is higher than the xBA of Paul LoDuca even though LoDuca's bb%, ct% and particularly eye are much better than Hernandez's.

I have three questions. How is xBA calculated or at least which variables are included? How much, if at all, does eye influence that calculation? Also, I had assumed that the xBA shown in the Playerlink database was based only on that season's underlying stats. Is that the case or does it also include historical data from previous years?
Thanks.

[Tom_T]

It's probably not obvious, but xBA is in the Glossary under "Expected":

http://www.baseballhq.com/overview/glossary/xBA/index.html

[kahudson]

Thanks, I feel a little like a dunce. I looked at BA and xERA, but not at expected. Not knowing the ground ball ratios and the like for LoDuca and Hernandez, I can assume that part of the explanation has to do with the significantly higher PX of Hernandez.

[Michael@HQ]

I think John Burnson introduced a different formula for xBA, so the definition in the glossary might not always be the one that is used.