Abstract: We included 25 studies of which 4 are awaiting assessment. The 21 studies that could be analysed were 1 RCT, 14 cRCTs and 6 CBAs with a total of at least 3479 employees in intervention and control groups. We judged 12 studies to have an unclear risk of bias and the remaining nine studies to have a high risk of bias. Sixteen studies focused on improving supervisor-employee interaction, whereas five studies aimed at improving the design of working environments by means of supervisor training. Training versus no intervention We found very low-quality evidence that supervisor training does not reduce employees' stress levels (6 studies) or absenteeism (1 study) when compared to no intervention, regardless of intervention type or follow-up. We found inconsistent, very low-quality evidence that supervisor training aimed at employee interaction may (2 studies) or may not (7 studies) improve employees' well-being when compared to no intervention. Effects from two studies were not estimable due to missing data. Training versus placebo We found moderate-quality evidence (2 studies) that supervisor training off the job aimed at employee interaction does not reduce employees' stress levels more than a placebo training at mid-term follow-up. We found low-quality evidence in one study that supervisor training on the job aimed at employee interaction does not reduce employees' absenteeism more than placebo training at long-term follow-up. Effects from one study were not estimable due to insufficient data. Training versus other training One study compared the effects of supervisor training off the job aimed at employee interaction on employees' stress levels to training off the job aimed at working conditions at long-term follow-up but due to insufficient data, effects were not estimable. Based on a small and heterogeneous sample of controlled intervention studies and in contrast to prevailing consensus that supervisor behaviour influences employees' health and well-being, we found inconsistent evidence that supervisor training may or may not improve employees' well-being when compared to no intervention. For all other types of interventions and outcomes, there was no evidence of a considerable effect. However, due to the very low- to moderate-quality of the evidence base, clear conclusions are currently unwarranted. Well-designed studies are needed to clarify effects of supervisor training on employees' stress, absenteeism, and well-being.

Plain-language summary: We included 25 studies of which 4 studies are awaiting assessment. The 21 studies that could be analysed included a total of 3479 employees. Sixteen studies trained supervisor-employee interaction, either off-the-job (9 studies) or on-the job (7 studies). Five studies trained the design of working environments, off-the-job in 2 studies and on-the job in 3 studies. The 21 studies compared 23 interventions with no training, sham training or other training at various times of follow-up. There is no considerable effect of supervisor training on employees' stress (6 studies) or absenteeism (1 study) when compared to no training. There is inconsistent evidence that supervisor training may (2 studies) or may not (7 studies) improve employees' well-being when compared to no training. Data were missing from two studies, so we could not calculate the effects of training on employee well-being. There is no effect of supervisor training on employees' stress (2 studies) or absenteeism (1 study) when compared to a placebo training. Data were missing from one study, so we could not calculate the effects of training on employee well-being. One study that evaluated supervisor training compared to another type of training to reduce employees' stress did not provide enough data to calculate its effects. The quality of the evidence was very low for most outcomes due to risk of bias in the studies, inconsistent results, and imprecise effects. Researchers should consider the shortcomings of studies included in this review in order to conduct well-designed studies in the future and report them appropriately. Overall, the data suggest that training of supervisors may not lead to reduced levels of stress and absenteeism, or improved levels of well-being in their employees. The discrepancy between the apparent scientific consensus and the empirical evidence might be attributed to weak study designs.High quality studies are needed to clarify if supervisor training affects employees' stress, absenteeism, and well-being.