Re: "changes in perceived job strain and the risk of major depression: results from a population-based longitudinal study".

We read with interest a recent article in the American Journal of Epidemiology by Wang et al. (1), examining changes in job strain and the risk of major depression. Research describing how change in psychosocial working conditions impacts mental health status is needed to increase the awareness among policy makers and employers of the possible health ramifications of changing workplace organization and practices. However, the measurement of change between time points among individuals is a complex task (2–4), one which we feel that Wang et al. (1) may not have addressed with sufficient care. Similar to some others (5–7), Wang et al. (1) used the crossing of a threshold to indicate when respondents had ‘‘changed’’ in job strain. In their defense, they used a relatively meaningful cutpoint, that is, where job demands exceeded job control, an improvement on the use of an arbitrary cutpoint based on the median score. However, does a simple cutpoint allow us to know who has changed and who has not? Consider the following example. If both job control and psychological demands are scored on a scale of 1–21 points, the job strain ratio may stretch from 0.05 to 21 points, with ‘‘1’’ being the point where demands exceed control. Figure 1 describes changes from 7 hypothetical individuals (A–G). One would be hard pressed to conclude that individual D (or even individual F) in Figure 1 has changed more than individuals A, E, and G, although the analysis by Wang et al. (1) would identify individuals B, D, and F as changed (the black lines) but individuals A, C, E, and G (the gray lines) as not. This highlights the distinction between analyzing who has changed as inferred in the title of the article by Wang et al. (1) and examining who ends up beyond a particular threshold (which was actually done). Change in job strain could be defined as any movement in the job-strain ratio beyond the expected variability in the instrument when no actual change has occurred (estimated by using the bounds of error). We have previously reviewed and applied methods to undertake this process for job control (8, 9) in the same data set as used by Wang et al. (1). Returning to Figure 1, if individual D’s job-strain change is no more than day-to-day variability, can we really define him/her as having changed, regardless of the threshold being crossed? Others suggest combining both change and final state to identify changes that are important (10, 11). Under this approach, we would want to know who has changed beyond error and who has moved beyond a score of 1. As such, the results of Wang et al. (1) tell us that moving across a job-strain threshold of 1 is related to depression onset but, for the reasons above, they cannot be extended to infer that change in job strain affects depression. We also encourage Wang et al. (1) to examine if the effects on depression for crossing their job-strain threshold (and ideally also for change in job strain) are still present when occupation does not change between time points. Although a change in occupation will almost certainly result in a change in job strain, one might argue that changes in job strain, within occupation, provide the most convincing evidence to intervene in the psychosocial work environment, as an intervention that requires people to change jobs is commonly not feasible. We have previously reported that, although positive changes in job control are associated with lower levels of psychological distress in this sample, they are no longer present when the sample is restricted to individuals who did not change occupations (9), suggesting that lower psychological distress levels were due to changes in occupation, which in turn caused changes in level of job control. We applaud Wang et al. (1) for moving examinations of job stress and mental health forward but caution that, as research investigating the health effects of changes in working conditions increases, there is a need for greater attention to be given to the methodological issues involved in the analysis of change among individuals over time.