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False positives.
Regardless of which
estimates one believes, only a small frac-
tion of adults have used guns defensively
in 1994. The only question is whether
that fraction is 1 in 1,800 (as one would
conclude from the NCVS) or 1 in 100
(as indicated by the NSPOF estimate
based on Kleck and Gertz's criteria).
Any estimate of the incidence of a rare
event based on screening the general
population is likely to have a positive
bias. The reason can best be explained
by use of an epidemiological frame-
work.
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Screening tests are always sub-
ject to error, whether the "test" is a
medical examination for cancer or an
interview question for DGUs. The er-
rors are either "false negatives" or
"false positives." If the latter tend to
outnumber the former, the population
prevalence will be exaggerated.
The reason this sort of bias can be ex-
pected in the case of rare events boils
down to a matter of arithmetic. Sup-
pose the true prevalence is 1 in 1,000.
Then out of every 1,000 respondents,
only 1 can possibly supply a "false
negative," whereas any of the 999 may
provide a "false positive." If even 2 of
the 999 provide a false positive, the
result will be a positive bias—regard-
less of whether the one true positive
tells the truth.
Respondents might falsely provide a
positive response to the DGU question
for any of a number of reasons:
• They may want to impress the inter-
viewer by their heroism and hence ex-
aggerate a trivial event.
• They may be genuinely confused due
to substance abuse, mental illness, or
simply less-than-accurate memories.
• They may actually have used a gun
defensively within the last couple of
years but falsely report it as occurring
in the previous year—a phenomenon
known as "telescoping."
Of course, it is easy to imagine the
reasons why that rare respondent who
actually did use a gun defensively
within the time frame may have de-
cided not to report it to the inter-
viewer. But again, the arithmetic
dictates that the false positives will
likely predominate.
In line with the theory that many DGU
reports are exaggerated or falsified, we
note that in some of these reports, the
respondents' answers to the followup
items are not consistent with respon-
dents' reported DGUs. For example, of
the 19 NSPOF respondents meeting
the more restrictive Kleck and Gertz
DGU criteria (exhibit 7), 6 indicated
that the circumstance of the DGU was
rape, robbery, or attack—but then re-
sponded "no" to a subsequent ques-
tion: "Did the perpetrator threaten,
attack, or injure you?"
The key explanation for the difference
between the 108,000 NCVS estimate
for the annual number of DGUs and
the several million from the surveys
discussed earlier is that NCVS avoids
the false-positive problem by limiting
DGU questions to persons who first re-
ported that they were crime victims.
Most NCVS respondents never have a
chance to answer the DGU question,
falsely or otherwise.
Unclear benefits and costs from
gun uses.