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RESEARCH LETTERS
Bias when using dead controls to study handgun purchase as a risk factor for violent death
  1. D J Wiebe,
  2. C C Branas
  1. Department of Biostatistics and Epidemiology/Firearm and Injury Center at Penn/Leonard Davis Institute of Health Economics, University of Pennsylvania, 933 Blockley Hall, Philadelphia, PA 19104-6021, USA; dwiebe@cceb.med.upenn.edu

    https://doi.org/10.1136/ip.9.4.381

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      In a recent, excellent study that found handgun purchase to be a risk factor for violent death, Grassel and colleagues gave explanations for why their findings may have inaccurately characterized the impact of handgun purchase on violent death, leading to results that if anything underestimate the magnitude of the danger associated with buying a handgun.1 “The healthy handgun purchaser effect”, which we suggest is akin to the healthy worker effect, may be a source of bias as well. Bias from this source could produce results that either underestimate or overestimate the impact of a handgun purchase on violent death. When dead controls are used in a case-control study, the death rate due to the comparison cause(s) should be equal in the exposed and unexposed populations.2–6 If this assumption is met, the odds ratio (OR) can be used as an estimate of the mortality density (or rate) ratio:

      Math

      where a and c are exposed and unexposed cases and b and d are exposed and unexposed controls, respectively, and TE and TĒ are the person-time contributed by exposed and unexposed subjects, respectively. We follow Morgenstern and use his notation,6 but notation used …

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