Background: Experience Sampling Methods (ESM) have been used in clinical research to collect data on emotional and behavioural states in real life contexts among different populations. Although the use of ESM in mental health has increased, it has not been applied to larger samples of young people in disadvantaged urban settings. Objective: This study aimed to determine the extent to which mood status scores (happiness and nervous) vary during a week as a function of having or not having symptoms of depression and/or anxiety in a sample of adolescents and young adults in the cities of Buenos Aires, Bogotá and Lima. A secondary objective was to identify factors associated with mood scores, including socio-demographics, quality of life, and daily activities. Methods: The study was part of the OLA research programme, which focuses on mental health resources for young people. Participants (n=143) aged 15-24 completed daily ESM assessments over a week using the mobile App, resulting in 5246 reports. Data were analysed using descriptive analyses with t-tests and chi-square, and multilevel linear regression was used to examine associations between depressive/anxiety symptoms, mood variability and factors influencing mean mood. Finally, Spearman's correlation assessed the relationship between happiness and nervousness. Results: The analysis revealed that depressive and/or anxiety symptoms were not significantly associated with increased variability in mood scores (happiness p=0.396, nervousness p=0.839). However, males exhibited greater variability in happiness and nervousness scores (p

Mok Sze Wai, Cindy, EdD student, HKU, takes us through some of the key issues in using experience sampling method (diary method) as a longitudinal research methodology.