This paper presents the development and evaluation of the train-the-trainer (TTT) workshop for lay resident leaders to be lay health promoters. The TTT workshop aimed to prepare the trainees to implement and/or assist in conducting a series of community-based family well-being activities for the residents in a public low rent housing estate, entitled "Learning Families Project", under the FAMILY project. The four-hour TTT workshop was conducted for 32 trainees (72% women, 43% aged ≥ 60, 41% ≤ elementary school education). The workshop aimed to promote trainees' knowledge, self-efficacy, attitude and practice of incorporating the positive psychology themes into their community activities and engaging the residents to join these activities and learn with their family members. Post-training support was provided. The effectiveness of the TTT was examined by self-administered questionnaires about trainees' reactions to training content, changes in learning and practice at three time points (baseline, and immediately and one year after training), and the difference in residents' survey results before and after participating in the community activities delivered by the trainees. The trainees' learning about the general concepts of family well-being, learning family, leadership skills and planning skills increased significantly with medium to large effect sizes (Cohen's d: 0.5-1.4) immediately after the training. The effects of perceived knowledge and attitude towards practice were sustained to one year (Cohen's d: 0.4-0.6). The application of planning skills to implement community activities was higher at one year (Cohen's d: 0.4), compared with baseline. At one year, the residents' survey results showed significant increases in the practice of positive communication behaviours and better neighbour cohesions after joining the family well-being activities of LFP. Qualitative feedback supported the quantitative results. Our TTT workshop could serve as a practical example of development and evaluation of training programs for lay personnel to be lay health promoters.
Trial registration: ClinicalTrials.gov NCT02844244.