Police Clearance Certificates

Hi,
I'm hoping some of you might be able to help me out. I've surfed through this site and others and I have found conflicting information. Can you tell me if Police Clearance Certificates from country of origin are always required? If yes, can you tell me if someone were to have a conviction (a Summary conviction for example) from their country of origin, whether this would necessarily prevent them from obtaining a work/residence visa for Qatar?
Thanks so much!
Just for practical thinking, why would it be required for a police clearance? It is the way to find out if you are a clean person, but in many cases, they do give a chance to some convicted persons, dependeng to what jobs being offered. And it should be the way, in my opinion because it is ones human rights.
"There's nothing we can do to change the past, if it teaches you a lesson profit from it then, forget it."
It is needed and you will be asked for it when submitting your papers. About a conviction I don't know can't help you here.