Ten studies were included, of which seven were controlled. The overall number of participants was not stated.
The overall quality of studies was judged as relatively high. Most assessed the reliability of diagnoses at baseline and also evaluated the degree to which the treatment protocol was maintained. There were high attrition rates among studies of older participants.
Immediate effectiveness of CBT (seven controlled studies)
Pooling of seven controlled studies resulted in an overall ES of -1.15, significantly favouring CBT compared to controls receiving a non-specific active treatment or no current treatment (Fischer's z test p<0.05). The Fail Safe N test indicated that 18 studies with no effect would be required to negate the statistical significance of this result. However, there was significant statistical heterogeneity (p<0.05). Subgroup analysis by age showed a larger ES for young adults (mean age 39) (ES -1.69, standard deviation (SD) 0.36, p<0.05) than for older participants (mean age 68) (ES -0.82, SD 0.43, p<0.05). Hedges' test for homogeneity was non-significant for these subgroup analyses (p>1).
Effectiveness of CBT over time (eight studies)
When scores in the CBT groups at the end of treatment were compared with their measures at six and 12 months the differences in ES were quite small, indicating that the benefits of therapy were generally maintained over follow up. At six months, mean ES in the young adult group was 0.009 (SD 0.26) and in older adults it was 0.12 (SD 0.22). At 12 months, ES in the young adult group was 0.027 (SD 0.37) and in older adults it was 0.23 (SD 0.12). The mean PSWQ score of CBT groups after treatment was within the normal range in both young people (mean score 46.93) and older adults (mean score 51.00) and over one SD outside the clinical range. This trend continued over 12-month follow up.
Effect of setting
Subgroup analysis showed that the immediate effect of CBT was higher in studies using individual CBT (ES -1.72) than in those using group sessions (ES 0.91). Follow-up data indicated that studies using group sessions had a continued reduction in worry at six months (0.25) and 12 months (-0.43). In groups receiving individual therapy there was little change during follow up.