I'll let you all chew on this...60% of convicted felons admitted that they avoided committing crimes when they knew the victim was armed. 40% of convicted felons admitted that they avoided committing crimes when they thought the victim might be armed.
James Wright and Peter Rossi, Armed and Considered Dangerous: A Survey of Felons and Their Firearmsā, New York: Aldine, 1986
74% of felons agreed that "one reason burglars avoid houses when people are at home is that they fear being shot during the crime." 57% of felons polled agreed, "criminals are more worried about meeting an armed victim than they are about running into the police."
U.S. Bureau of Justice Statistics Federal Firearms Offenders study, 1997. Department of Justice, National Institute of Justice, "The Armed Criminal in America: A Survey of Incarcerated Felons," Research Report, July 1985
Check out the Gun Facts guide available at Gun Facts - Gun Control | Facts | Debunk | Myths, especially the section on deterrence at pages 10-11.