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Fujishiro et al1 have recently published data on the association of job demands and control with carotid artery intima-media thickness (IMT). The combined effect of demands and control (strain) was analysed using five different strain definitions:
a quadrant term (median splits of demands and control),
combinations of tertiles of demands and control,
an additive term (demands minus control),
a quotient term (the ratio) and
a multiplicative term (the product).
The first three terms are linear combinations of demands and control, which are less informative than the corresponding linear combination based on regression analyses of the mutually adjusted effects of demands and control. The quotient term implies interaction between demands and control but does not examine if there is one, its size, form or statistical significance. An effect of any of the first four strain terms may be due to an effect of only one of the two factors. Why introduce a strain measure of the combined effect of demands and control, if it may only reflect the effect of one of these variables?
A parsimonious and informative way to examine the combined effect of demands and control is regression analyses with demands, control and their multiplicative term included as covariates. The authors published the effects of the multiplicative term but not the main effects, which are needed to evaluate the form of any interaction. The authors only illustrated the form of the interaction using dichotomous combinations of demands and control.
The authors interpreted the interaction as confirmation of the job strain theory because high job control protected against thick IMT, especially among persons with high job demands. However, they overlooked that high job demands also protected against thick IMT. Thus, the interaction effect as a whole was not in accordance with the job strain model.
Provenance peer review Not commissioned; internally peer reviewed.