Twenty-seven RCTs (n=6,496, range 12 to 1,045) were included in the review. Sixteen RCTs (17 study arms) were universal school-based programmes (n=4,671), eight were indicated school-based programmes (n=1,183) and three were selective school-based programmes (n=642). Three RCTs scored a quality rating of 3 (one universal and two indicated school-based programme studies); 17 scored 2 and six scored 1; just under half of the studies reported random allocation, 52% had no follow-up data past post-test. Where reported, follow-up duration ranged from four to 36 months.
Twenty-one of 27 studies (78%) reported statistically significant improvements in symptoms of anxiety at post-test and/or follow-up (effect size range 0.11 to 1.37). Evaluation by intervention type showed conflicting findings for indicated interventions.
Eleven of 16 (69%) of universal interventions reported statistically significant improvements at post-test (effect size range 0.31 to 1.37) and two also reported significant improvements at follow-up. One other study that had not reported significant improvements at post-test reported a significant improvement at follow-up. Two selective interventions showed significant improvements at post-test (effect size 0.11 and 0.39), but did not report follow-up data.
Further comparisons showed that 50% of studies (n=3) delivered to children compared to 81% of studies (n=12) delivered to adolescents showed significant improvements in anxiety. The other comparisons indicated that the significant effects were not dependent on intervention content, delivery agent or type of control.