Background: Recent epidemiologic studies have estimated little or no increased risk of automotive crashes related to cell phone conversations by the driver, whereas earlier case-crossover studies estimated the relative risk as close to 4. Did earlier studies introduce a positive bias in relative risk estimates by overestimating driving exposure in control windows?
Methods: Driving exposures in a “control” window and a corresponding “case” window on the subsequent day were tabulated across 100 days for 439 GPS-instrumented vehicles in the Puget Sound area during 2005–2006.
Results: For control windows containing at least some driving, driving exposure was about one-fourth that of case windows. Adjusting for this imbalance reduces relative risk estimates in the earlier case-crossover studies from 4 to 1.
Conclusion: Earlier case-crossover studies likely overestimated the relative risk for cell phone conversations while driving by implicitly assuming that driving during a control window was full-time when it may have been only part-time.
From the Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, Detroit, MI.
Submitted 14 February 2011; accepted 9 September 2011; posted 14 November 2011.
The author reported no financial interests related to this research.
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Correspondence: Richard A. Young, Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, 9B-19 University Health Center, 4201 St. Antoine, Detroit, MI 48201. E-mail: firstname.lastname@example.org.