The editorial group responsible for this previously published document have withdrawn it from publication.
Reason for withdrawal
The authors have requested this Protocol to be withdrawn. The reason is that the protocol is out of date and does not meet the current methodological standards of The Cochrane Collaboration. A new protocol will be published in 2014. To view the published versions of this article, please click the 'Other versions' tab.
Protocol first published: Issue 2, 1999