GPower 3

G*Power version 3, a free program for power analysis, has just been released - it's a big step up from version 2, which looked a bit ancient, but still worked well. You can download it from here: http://www.psycho.uni-duesseldorf.de/abteilungen/aap/gpower3/ . You can read more about power here: http://www.jeremymiles.co.uk/misc/power