Sorry if this is old news but I have just come across this and it looks very useful. Philip. http://www.sciencedirect.com/science/article/pii/S1053811907011020 ******************************************************************** Philip Quinlan E-Mail: ptq1(a)york.ac.uk Department of Psychology FAX: (01904) 323181 The University of York Tel: (01904) 320000 Ext. 3135 Heslington Direct : (01904) 323135 York YO10 5DD U.K. ********************************************************************