226 Chapter 7 policies regarding employment of employees with a work disability facilitates the sustainable employment of these employees (23, 30). These types of policies may provide supervisors more time and resources for the guidance of employees with a work disability. The process evaluation in this study showed that about one third of the supervisors spend on average 4-7 hours more time on the guidance of employees with a disability after completion of the training. The extent to which companies provide supervisors extra time to spend on the guidance could play a role in the exact number of hours supervisors can spend on doing this. The process evaluation also showed that supervisors scored less positive on two factors, namely feedback and formal endorsement. These factors, which are not part of ‘Mentorwijs’, could hamper the implementation of ‘Mentorwijs’ in practice, and may explain the lack of effects on intention to adopt or applied behaviors. Strengths & limitations To our knowledge this is the first study that evaluates the effectiveness of a supervisor training to improve the guidance of employees with a work disability on the level of supervisors and employees, with a long-term follow-up period among employees. However, this study also contained several methodological limitations. First, the selection of employees with work disabilities was done by supervisors and might have resulted in selection bias. Supervisors may have selected a “better” employee to participate in this study. This might have biased the effects of ‘Mentorwijs’, in which the training may be less effective than our results suggest. Second, a small sample size of employees could also have biased the results and may have contributed to only finding significant effects at 8 months. Third, the control group of employees with a work disability was identified in other regions than the intervention group, and the allocation to the intervention group was not randomized. To address this limitation, we used a propensity score matching method to achieve optimal comparability between the groups in terms of primary outcomes measures and additional matching criteria (31). This allowed to control for major confounding variables, such as age, gender, and employment characteristics. Although, this does not exclude that unobserved or unmeasurable factors, such as type of work disability, organizational culture, and HR-policies, might have influenced our results and may have reduced the comparability between the intervention and control group. Fourth, selection bias might also have occurred in the group of supervisors that were followed over time. Supervisors already scored relatively high on certain behavioral outcomes. This may reflect that supervisors, who participated in this study, already had a more positive attitude towards the guidance of employees with a work disability, and therefore placing a limitation on the potential improvement of these measures. Another limitation is that the evaluation among supervisors did not contain a control group, which cannot totally exclude that intervention effects were caused by elements other than the training itself. Moreover, recall bias may also have
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