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Re: Working Permanent Night Shifts and Injury Risk (Underpowered Result?)

The current research paper by Fransen et al. (2006) contributes interesting and useful data to the emerging research area examining the potential risk of extended working hours, unusual work patterns, and the occurrence of a work related injury.

I am concerned, however, that readers may conclude that working permanent night shifts carries no increased risk of work injury, despite contrary evidence as they cited. Highlighted in their report (including the Abstract and Main Messages) was their conclusion that “Permanent night work is not associated with increased risk of work injury after adjusting for these other risk factors.”

In Table 4, the reported point estimate (RR) and 95% confidence interval (CI) for permanent night shift was 1.38 (0.95-2.00). I would argue that this “mutually adjusted result” suggests a fairly strong association between working permanent night shifts and the risk of a work injury; however, since the number of injured cases in this cell was small (n=69), this result is a potential Type II error and possibly statistically underpowered result (Freiman, J.A., T.C. Chalmers, H. Smith et al., 1978). Thus, it should have been interpreted with caution rather than suggesting with certainty that there is no association.

Additionally, it would be of interest to see the adjusted results when using working hours as a continuous variable, rather than a single cut point of 40 hours, which may or may not reflect long working hours.

Again, I am grateful to the authors for publishing these important results.

Fransen M, Wilsmore B, Winstanley J, Woodward M, Grunstein R, Ameratunga S, Norton R Shift work and work injury in the New Zealand Blood Donors' Health Study. Occup Environ Med 2006 May; 63(5) :352-8.