Survey Research and Self-Defense Gun Use: An Explanation of Extreme Overestimates

Abstract

Two aspects of the K-G survey combine to create severe misestimation.The first is the likelihood of positive social desirability response, sometimes referred to as personal presentation bias.An individual who purchases a gun for self-defense and then uses it successfully to ward off a criminal is displaying the wisdom of his precautions and his capability in protecting himself, his loved ones, and his property.His action is to be commended and admired.Some positive social desirability response bias, by itself, might not lead to serious overestimation.However, combined with a second aspect of the survey-the attempt to estimate a very rare event-it does.The search for a "needle in a haystack" has major methodological dangers, especially where researchers try to extrapolate the findings to society as a whole.Until the K-G study, no one had estimated that even as many as 1% of adult civilians had used a gun in self-defense in the past year.Nevertheless, assume that the actual incidence is 1%.On average, for every 100 individuals asked a 'Yes/No" question about the event, ninety-nine respondents will have a chance to be misclassified as a false positive.In ninety-nine answers there is the possibility of positive social desirability response bias.However, on average only one respondent-the one who actually did use a gun in self defense-could possibly be misclassified as a false negative (e.g., if she forgot about the event).Even if the chance of forgetting is high, as long as there is any possibility of positive response bias, it is very likely that the survey finding will be an overestimate.The fact that the survey is trying to estimate a low probability event also means that a small percentage bias, when extrapolated, can lead to extreme overestimates.Consider a survey finding which contains a 1% overestimate of positive responses.If the true incidence of the event is 60%, estimating it at 61% would not be a problem.But if the true incidence is 1%, measuring it as 2% would be a doubling of the true rate; and if the true incidence is 0.1%, measuring it at 1.1% would be an eleven-fold overestimate.The K-G survey design contains a huge overestimation bias.The authors do little to reduce the bias or to validate their findings by external measures.All checks for external validity of the Kleck-Gertz finding confirm that their estimate is highly exaggerated. II. BACKGROUNDPrevious data on self-defense gun use came from two sourcesthe large National Crime Victimization surveys (NCVS), and smaller private surveys (principally random-digit-dial telephone surveys).These two sources produce markedly different results.

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