Introduction
Prospective follow-up studies provide valuable insight into the impact of a condition or treatment on patients’ lives. Participants staying in a study is called ‘retention’. Retention is essential to follow-up studies. The longer the follow-up time, the more difficult it is to maintain satisfactory retention. The reasons why participants discontinue participating in a study might be because they are busy, have difficulties coming to the clinic, or are just unwilling to contribute any longer. Low retention leads to missing data, which can cause a bias and limit the generalizability, validity, and reliability of the results. It has been considered that < 5% loss of participants is not problematic, but a loss of > 20% is a serious threat to the validity of the study [A recent Cochrane review identified 70 studies that evaluated interventions to improve trial retention [This study aimed to investigate whether sending a postal pre-notification would increase the return rate of approval forms or questionnaires and reduce the number of post-notifications needed in 17-year-old adolescents. We hypothesized that the pre-notifications would increase both response rates (approval forms and questionnaires) and reduce the need for post-notifications.