There are instances where not only have warning shots failed, but actual shots into a perpatrator failed to stop a second unseen or un-noticed assailant from shooting the targeted victim.
Even if there were statistics on this what statistic would be deemed acceptable?
If warning shots succesfully "scared off" attackers 75% of the time? Ninety percent of the time? Ninety five percent of the time? If I knew there was a 10% chance of taking a bullet by warning I wouldn't but if it's only 5% I will?