Re:Analysis of job strain effects
We thank Dr. Mikkelsen and colleagues for their constructive comments on our paper. Our responses to their three major questions are listed below.
1) Why did we present various formulations of job strain?
The five formulations of job strain have been commonly reported in the literature. Often, authors chose one or two formulations and thus would not know if their results were consistent across different formulations. In our study, with the encouragement of an OEM peer reviewer, we presented results for all five formulations so that readers can compare the results both across different formulations and with previous job strain studies. We agree with Mikkelsen et al. that four of the five formulations of job strain we examined could be the result of the effect of job control only. As shown in Table 3, job demands was not significantly associated with IMT whereas job control was.
2) Why did we not show the main effect coefficients for job demands and control as we showed the coefficient for the multiplicative term in Table 4?
When a multiplicative term is included in a regression model, the main effects of the interacted variables have a more complex interpretation;1 that is, the coefficients represent the magnitude of effect for each variable when the other is 0. We showed the main effects of the two variables in Table 3, which represent the magnitude of each variable's effect when the other is controlled for. For the sake of brevity, we did not show the coefficients for demands and control after the multiplicative term was included in the model. This was explained in the footnote for Table 4.
3) Why did we illustrate the interaction in dichotomous terms (Figure 1) and ignored that high job demands were protective?
Figure 1 is an illustration of the interaction, which could have been constructed using the mean+1SD as "high" and the mean-1SD as "low" or other ways. We used the median split again for the sake of simplicity and also because it is a commonly used approach in the job strain literature. We do recognize that the demand-control model was only partially supported in our study; that is, job control was protective only for those who reported high job demands. Overall, job demands did not have a significant association with IMT, as shown in Table 3. The following paragraph is our discussion on this finding from an earlier draft. Unfortunately, this paragraph was excluded from the final version because of the word limit.
Contrary to the demand-control model's prediction,2 we did not find significant associations between IMT and job demands. The Cardiovascular Risk in Young Finns Study also failed to find the same significant association.3 The majority of CVD studies have found positive associations between job demands and CVD,4 but Belki? et al.4 identified five studies that reported an inverse association between psychological job demands and CVD.5-9 In the MESA cohort, of which about 30% were immigrants, the job demands scale had acceptable scale reliability among U.S.-born participants but not among immigrants (Cronbach's alpha = 0.75 for U.S.-born, ranged from 0.45 to 0.65 for immigrants, depending on the language used in data collection10). Limitations in the measure of job demands used in heterogeneous samples like ours may have limited our ability to detect associations of job demands with IMT.
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10. Fujishiro K, Landsbergis P, Diez Roux AV, Hinckley Stukovsky K, Shrager S, Baron S. Factorial invariance, scale reliability, and validity of the decision latitude and psychological demands scales for immigrant workers: The Multi-Ethnic Study of Atherosclerosis (MESA). Journal of Immigrant and Minority Health 2010;13:533-40.
Conflict of Interest:
Analysis of job strain effects
Fujishiro et al.1 recently published data on the association of job demands and control with carotid artery intima-media thickness (IMT). The joined effect of demands and control (strain) was analyzed by five different strain definitions: 1. a quadrant term (median splits of demands and control), 2. combinations of tertiles of demands and control, 3. an additive term (demands minus control) , 4. a quotient term (the ratio) and 5. 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 joined 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 joined 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. These are needed to evaluate the form of any interaction. The authors only illustrate the form of the interaction by dichotomous combinations of demands and control. The authors interpret 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 overlook that high job demands also protected against thick IMT. The interaction effect as a whole was not in accordance with the job strain model.
References 1. Fujishiro K, Diez Roux AV, Landsbergis P, et al. Associations of occupation, job control and job demands with intima-media thickness: The Multi-Ethnic Study of Atherosclerosis (MESA). Occup Environ Med 2011;68:319-326.
Conflict of Interest:
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