onstat & vmstat CPU measurements

Greetings, I'm trying to calibrate CPU utilisation on our system using onstat, "ps -el" (both updated before and after a test job) and vmstat (updated every second). onstat's view of CPU utilisation appears to be pretty much consistent with that reported by "ps -el", but these are reporting higher values than those reported by vmstat. I have taken into account that we are executing on a 6-way box and that the vmstat averages across all 6 engines. Any clues ? Regards, j