Please ban this guy from posting. He is spraying wrong info without proof.You need PCC from all countries where you lived for more than 6 months continuously - not as total. Shorter visitor trips doesn't matter. CIC can gather that easily from your passport stamps. Besides I94 should have your travel history end to end. And even if you don't have departure stamps from US, you must have had arrival stamps from whatever the country you are landing for sure right.
The correct info is on CIC website where as one required to have PCC for all countries where he/she spent 6 months or more accumulative after 18 years old.
http://www.cic.gc.ca/english/information/security/police-cert/intro.asp