Limitations
Process evaluations for each study, which would have enhanced our contextual understanding of the study outcomes, were not conducted. Future research investigating the effectiveness of behaviourally-informed letters should therefore undertake process evaluation to better understand how and why the interventions may be effective. Also, the trials were not designed to be compared. As such there are many differences between the two trials (i.e., setting, age group, and trial design) which make comparing the results from each trial to better understand the determinants of an effective letter intervention difficult.