Too much information missing. Depends on the position you are in the store, the degree of which you think they're likely to commit harm (their demeanor), the likelihood if I engaged of me injuring others, my likelihood of success, etc.
Knife? Unlikely I engage at all, unless they actually make an attempt to injure someone. Gun is a different story and involves all of the above.
Your goal is not to play hero; it's to survive and help others if you reasonably can. You can get yourself in much more trouble by responding and doing poorly than just letting it go and hoping for the best (that the clerk will hand over the money and the perp will exit quickly).
"You'll Never Walk Alone"
Last edited by david05111; 01-05-2013 at 12:55 AM.