Abstract
This report has been authorized by the Scientific Committee of the IPSM. The membership of the working party was: S C Klevenhagen (chairman), R J Aukett, J E Burns, R M Harrison, R T Knight, A E Nahum, and K E Rosser. Thanks are due to J E Burns for preparing the several drafts on which this report is based, and for originally drawing the attention of the IPSM to the need for such a report.