This presentation highlights the importance and implications of assessing the role of baseline wellbeing on intervention effectiveness and effect sizes when conducting wellbeing intervention studies. While disease severity is a well-established determinant of treatment success in a range of physical and mental illnesses, the impact of wellbeing level on intervention effectiveness is not widely researched. This presentation uses data from three studies conducted by the South Australian Health and Medical Research Institute (SAHMRI) Wellbeing and Resilience Centre, in which the effect of a general wellbeing training was assessed in improving wellbeing. The studies showed a consistent pattern: individuals with low wellbeing at baseline demonstrated significantly higher effect sizes (Cohen’s d ranging between 0.55 and 0.75) compared to those with high wellbeing at baseline (d ranging between 0.13 and 0.17), after completion of the training. As there is a differential effect based on baseline levels of wellbeing, and as low wellbeing is consistently associated with a range of negative outcomes (risk of developing mental illness, lower productivity, poorer school outcomes, to name a few), the case should be made for wellbeing screening before wellbeing interventions roll out, particularly when immediate impact and cost-effective implementation is desirable.