Conclusions
U54EB020404, Cancer
RECRUITMENT, ABUSE, CANCER
We found a large causal effect of sending notifications on near-term engagement. The probability of opening the app in the immediate hour increased 3.5-fold when receiving a notification compared with not receiving a notification. Notifications are important and effective components of behavior change apps; however, a policy of sending a fixed daily notification or a randomly chosen series of notifications did not increase the amount of engagement or length of time to disengagement for users compared with a policy of no notifications. This suggests that notifications may serve users better when they are implemented as dynamic components, such as sending a notification to increase the perceived usefulness of the app only when the users’ engagement patterns show that they are at risk of disengaging.Further optimization of the notification policy is required to improve long-term engagement. The next stage of research is to explore how our findings would help develop a policy for The development of the LB is supported by a Doctor of Philosophy studentship funded by the Medical Research Council (MRC) Network of Hubs for Trials Methodology Research (MR/L004933/2-R18). CG is funded by the NIHR (NIHR127651) and the NIHR School for Public Health Research and Cancer Research UK (CRUK: C1417/A22962). TQ is supported by the National Institute on Alcohol Abuse and Alcoholism of the National Institutes of Health under award R01AA23187, National Institute on Drug Abuse of the National Institutes of Health under award P50DA039838, National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health under award U54EB020404, and National Cancer Institute of the National Institutes of Health under award U01CA229437. The content is the sole responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. OP is funded by Cancer Research UK (CRUK: C1417/A22962). HWWP has received funding from Public Health England, the NIHR, and National Health Service (NHS) England. EW is funded by the Wellcome Trust (grant 224485/Z/21/Z) and MRC project grant (grant MR/S01442X/1). LB would like to thank the MRC Biostatistics Unit and Papworth Hospital for funding time spent on this research.Conflicts of Interest: CG is part of the team that developed and evaluated the 
The editor granted an exception from ICMJE rules mandating prospective registration of randomized trials, because the primary outcome is an engagement measure and not a health outcome. Readers are advised to carefully assess the validity of any potential explicit or implicit claims related to primary outcomes or effectiveness. However, note that the authors made publicly available the submitted preprint of the trial protocol (through Visual of Privacy notice.Information Sheet.Bank of 30 newly developed messages and their link to the relevant behavior change module.Sample size calculation.Recruitment plots.CONSORT-eHEALTH checklist (V 1.6.1).