Inv: courage to cut loss?

Nobody has the ability to win everytime. The problem is, to cut loss decisively or not? That is, to set a target, eg. 10%, if lose over the target, then sell the stocks decisively, immediately, no matter what reason behind?
I think it may help as we are not expert, and limit ability to do through study. Especially, most events aren't under our control.
Your view?