Deltoid

In other words, the NCS only counts defensive uses against crimes.

Andy Freeman said:

Wrong. NCS doesn't get into defensive uses unless the victim thinks that a crime occurred even if it was successfully self-defended against. As in "Have you been the victim of a crime?" Someone who successfully self-defended against an armed robber might well answer "no". They weren't a victim. Yet, that's a self-defense against crime, one that the NCS wouldn't ever find out about.

In fact the NCS does not ask "Have you been the victim of a crime?". Andy appears to have made that question up. The relevant questions to Andy's example are "Did anyone TRY to rob you?" and "Did anyone threaten you with a weapon?"

Let's suppose for the moment that Andy's claim is true, and that this explains the enormous discrepancy between the NCS estimate of defences (80,000), and the 1,000,000 figure that some people here seem to believe. What are the implications?

(1) The people who designed the NCS surveys are incompetent. NCS is supposed to measure attempted crimes, but only finds out 10% of them.

Wrong. That isn't an implication. NCS is interested in victims. These people don't necessarily consider themselves victims, and NCS isn't interested in finding the ones that don't.

Can you provide some evidence for this remarkable claim? Like an NCS document where they state "we're only interested in people who consider themselves victims". Or perhaps a screening question where they ask "do you consider yourself a crime victim?" OR did you make this one up too?

(2) NCS data on completion rates of crimes is utterly worthless since it is based on only 10% of attempted-but-not-completed crimes.

Wrong again. Some people may be victims even though the crime wasn't completed.

So? How does this address the point I raised?

People who successfully self-defend are not necessarily like everyone else who was a target of an unsuccessful attack.

Yes, so if completion rates are lower for "self-defence with a gun" you cannot infer that guns are more useful for self-defence.

(BTW - Consider the crime "attempted murder".)

Certainly. A victim survey on murder would discover that the completion rate for murder was 0%, irrespective of self-defence measures used. Because the survey is biased (towards people who are still alive) it tells us nothing useful about the success rate of would-be murderers. Similarly, if the NCS is, as you claim, biased away from successful defenders, it tells us nothing useful about crime completion rates.

NCS data can be quite useful for many purposes without being able to tell us about everything.

Certainly, but you want to reject the bits that do not support your political beliefs and accept the bits that do.