Health Psychology, Vol 44(6), Jun 2025, 578-586; doi:10.1037/hea0001436
Objectives: We tested whether an intervention’s dosage influences health behavior change. Intervention dosage was indexed by the duration of the intervention, the number of sessions, and total contact time in treatment arms of trials to promote physical activity. Method: The research comprised two studies. In Study 1, we synthesized 161 meta-analyses of physical activity interventions to examine the relationship between intervention dosage and intervention impact on physical activity. Study 2 comprised a reanalysis of primary studies. Results: The relationship between the indices of dosage and intervention effect sizes was assessed in 23.6% of meta-analyses, and associations did not differ from zero in 70%–85% of tests (Study 1). In Study 2, we retrieved 838 tests for duration, 370 tests for the number of sessions, and 167 tests for contact time. Duration had a very small negative association with effect sizes (b = −.002, 95% confidence interval [−.003, −.001]); number of sessions and contact time were not related to intervention effectiveness. An exploratory study (N = 50) indicated that health psychology researchers overestimated the association between dosage and effect sizes (restimates > .25). Conclusions: Although physical activity interventions with a higher dosage are expected to be more effective, the findings showed no relationship between dosage and intervention impact. Intervention dosage may moderate the influence of intervention content rather than directly determine physical activity change. (PsycInfo Database Record (c) 2025 APA, all rights reserved)