Permanent and fixed-term workers who are in the middle stage of their career did not differ on high-risk of job stress as measured by the ERI questionnaire. However, through detailed comparison of each ERI component, we found that these two types of employees suffer from different sources of occupational stress. Too much effort, such as overwork and job demand, seemed to distress permanent workers more while job insecurity seemed to distress fixed-term workers more. The same results were found even when we performed stratified analysis among clerical workers and researchers.
The proportion of workers in the high-risk ERI ratio group (ERI ≥ 1) was the same among permanent and fixed-term workers. Therefore, the seriousness of effort-reward imbalance of both permanent and precarious workers did not differ at all and both employment types experienced hardship from job stress. What differed between permanent and fixed-term workers were the sources of job stress. A greater proportion of permanent workers was distressed by the effort component compared to fixed-term workers. This result proved that permanent workers suffered distress from job demand, workload, and too much responsibility. These permanent workers experienced slight downsizing in this institute. Demand for permanent workers should therefore increase. We focused on a mid-career population in our study. These workers might experience a dilemma working with employees in senior positions in addition to working with younger workers.
Regarding reward subcategories, permanent workers complained more about esteem compared to fixed-term workers. Permanent workers were more distressed from not getting the respect from their seniors and colleagues based on our detailed analysis of each statement (not shown in the tables). Self-esteem of permanent workers who might perceive themselves superior might be mismatched. Our study subjects were middle-aged and had a moderate career, and the self-confidence of permanent workers might influence their perception of self-esteem. Even for fixed-term workers, this institution seems to have a supportive organizational culture because fixed-term workers with higher self-esteem experienced less job stress. A possible reason is that workers in research or academic fields usually start their job as precarious employees. This type of employment might not stigmatize their occupational status as much.
Job promotion, as the reward component, reflects satisfaction with the occupational position and income. No difference was found between the two groups on job promotion. However, in our detailed analysis of each statement, fixed-term workers were distressed more because they were less satisfied with the position and/or income compared to permanent workers. We focused on the workers in the middle stage of their career who might be concerned about their future position and salary to support their lifestyles and their families. Perception of the limited work prospect might influence hopelessness or discourage workers from continuous competition in the research occupation.
As frequently reported [29, 31], job insecurity was a cause of distress among fixed-term workers more than among permanent workers also in our study. One of the items assessing job security was "My job security is poor." For this question, 251 (52%) fixed-term workers answered "distressed" while only 31 (14%) permanent workers provided the same answer. Even small numbers of permanent workers felt job insecurity. This might be because permanent workers may be also afraid of downsizing or restructuring. The other item assessing job insecurity inquired whether workers experienced or expect to experience an undesirable change in their work situation. Permanent workers were more likely to have experienced an undesirable change in one of the statements of the job security sub-category in ERI (permanent workers 44% and fixed-term workers 25%). This might be because permanent workers tended to work at the same institution for a longer period and realize when small changes occurred. One possible undesirable change could be replacing permanent workers with fixed-term workers.
High ERI and its association with subjective symptoms and obesity were suggested for fixed-term workers. It seems that a high ERI of fixed-term workers was more likely linked to physical or mental symptoms or obesity. Association with high ERI and poor health was concordant with previous studies  and we provided the evidence for that in particular employment status. Permanent workers were more obese in our analysis while higher ERI was associated with fixed-term workers' obesity. Fixed-term workers who have high job stress might resemble permanent workers regarding their health. In addition, obese fixed-term workers might experience physical weight gain from job stress, which might lower workers productivity and decrease the likelihood of promotion or finding a permanent position. Previous study suggested that obesity could be an obstacle to getting permanent work . A similar tendency concerning obesity and subjective symptoms might be applicable to our study subjects.
Several limitations of our study should be noted. Since our study was a cross-sectional study, causal relationship cannot be established. However, this study at least describes the characteristics and sources of job stress among different types of employees. Second, this study was conducted in one institution in Japan. Generalizing our results to other organizations may be difficult and must be done with caution. The effort/reward ratios, 0.58 for permanent workers and 0.48 for fixed-term workers, were similar to a previous Japanese study conducted among men in which the ratio was 0.5 [30, 33]. Thus, our study subjects were not an extreme population. Conducting the study in one institution could be advantageous as it can capture a population with workers of different employment status engaging in the same work and working in the same environment. Third, as our results for the esteem component showed that the targeted institution provided its employees with a supportive and fair environment. Even fixed-term workers obtained support from their colleagues, suggesting that inequity in the working environment might be small. This fair environment might underestimate the general environment of Japanese precarious workers. Although the targeted institution had a supportive environment, we found that workers actually felt job stress. It might be that personality characteristics influence perception of their working environment. In addition, the current study was conducted in a research institute where even precarious workers might easily control their workload. Workers in routine jobs reported that they could not control their work hours and tasks . Examining other job categories might provide different results. Finally, we could not gather information on personal background, such as educational background, history of job career, and income. However, regardless of employment types, researchers attained at least graduate degrees, and clerical workers had at least some university level education. Thus, the educational attainment between permanent and fixed-term workers in our study may not be different. In spite of these limitations, our study evaluated workers' stress, particularly job stress, using ERI and described the association between a higher ERI and the health of permanent and fixed-term workers.