Psychologically healthy participants may dilute the observed effects of worksite stress management training (SMT) programs, therefore hiding the true effectiveness of these interventions for more distressed workers. To examine this issue, 311 local government employees were randomly assigned to SMT based on acceptance and commitment therapy (SMT, n = 177) or to a waitlist control group (n = 134). The SMT program consisted of three half-day training sessions, and imparted a mixture of mindfulness and values-based action skills. Across a 6-month assessment period, SMT resulted in a significant reduction in employee distress. As predicted, the impact of SMT was significantly moderated by baseline distress, such that meaningful effects were found only among a subgroup of initially distressed workers. Furthermore, a majority (69%) of these initially distressed SMT participants improved to a clinically significant degree. The study highlights the importance of accounting for sample heterogeneity when evaluating and classifying worksite SMT programs.
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