You can find the study here:
Armed Resistance to Crime: The Prevalence and Nature of Self-Defense with a Gun - Journal of Criminal Law & Criminology, Volume 86, Number 1, 1995
More specifically, toward TurtleDude's comment about incarcerated criminals fearing armed homeowners, from the above study:
74% of felons polled 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 that "criminals are more worried about meeting an armed victim than they are about running into the police."
Armed citizens prevent more than 2.5 million crimes every year, and are responsible for killing more criminals than the police.