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Case-crossover studies of occupational trauma: methodological caveats

¹ Liberty Mutual Research Center for Safety and Health, Hopkinton, Department of Environmental Health, Occupational Health Program, Harvard School of Public Health, Boston, and Department of Biostatistics and Epidemiology, University of Massachusetts, School of Public Health and Health Sciences, Amherst, Massachusetts
² Department of Environmental Health, Occupational Health Program, Harvard School of Public Health, Boston, and Department of Biostatistics and Epidemiology, University of Massachusetts, School of Public Health and Health Sciences, Amherst, Massachusetts
³ Department of Allied Health, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin
⁴ Center for Injury Research and Policy, Johns Hopkins University, School of Hygiene and Public Health, Baltimore, Maryland
⁵ Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts

Correspondence to:
Dr Gary S Sorock, Senior Researcher/Epidemiology, Liberty Mutual Research Center for Safety and Health, 71 Frankland Road, Hopkinton, MA 01748, USA
gary.sorock{at}libertymutual.com

Objectives—The case-crossover study design was developed to examine triggers for the onset of myocardial infarction. This paper seeks to examine selected methodological issues when applying the case-crossover method to the study of traumatic injuries in the work environment.

Methods—Researchers known to be working on occupational case-crossover studies were invited to present at a workshop held at the National Occupational Injury Research Symposium in October 2000. Data from ongoing studies were used to illustrate various methodological issues involved in case-crossover studies of occupational injury.

Key findings and issues identified—To utilize the case-crossover design, investigators must clearly define the time during which a worker is at risk of injury, the period of time during which a particular transient exposure could cause an injury and carefully select control time periods that estimate the expected frequency of exposure. Other issues of concern are changing work tasks over time, correlated exposures over time and information bias.

Conclusions and future research needs—More case-crossover studies of occupational injury are needed to compare results from multiple studies. The validation of the timing of transient exposures relative to injury onset, whether done in a laboratory or field setting, should be conducted. Nested case-crossover designs in other epidemiological studies (case-control or cohort) can examine both transient and fixed risk factors for occupational injury, and should be attempted.

Keywords: case-crossover; methodology; occupational injury

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