Statistical findings of the ten different studies of various durations were converted to "d" statistics and plotted against the duration of study. The results showed an increase in the effect size of guided imagery over the first 5 to 7 weeks. However, it appeared that the effect was decreased at 18 weeks.
Citation: Van Kuiken D. A meta-analysis of the effect of guided imagery practice on outcomes. Journal of Holistic Nursing. 2004 Jun; 22 (2): pages 164-79.