There were two earlier threads on this topic: http://finzi.psych.upenn.edu/R/Rhelp02a/archive/17554.html http://finzi.psych.upenn.edu/R/Rhelp02a/archive/10706.html Jon -- Jonathan Baron, Professor of Psychology, University of Pennsylvania Home page: http://www.sas.upenn.edu/~baron R search page: http://finzi.psych.upenn.edu/