There was an error in the alpha/a conversion in both the text and figure legends in the online version of this article from the time it was first available until May 2, 2006. Throughout the studies described in this article, "alpha" factor was used not "a" factor. We apologize for any confusion this error may have caused.
CORRECTION
10 May 2006
There was an error in the alpha/a conversion in both the text and figure legends in the online version of this article from the time it was first available until May 2, 2006. Throughout the studies described in this article, "alpha" factor was used not "a" factor. We apologize for any confusion this error may have caused.
Competing interests
NONE