Testing the applicability of idionomic statistics in longitudinal studies: The example of ‘doing what matters’

This study evaluated idionomic methods for identifying within-person links between therapeutically relevant processes and outcomes, using an ecological momentary assessment dataset of valued action and hedonic well-being (participants (n) = 425; 71.76% female; age = M(SD) = 22.20 (6.85); sampling design: 3–4 prompts per day; total measurements (n) = 6456). We compared the idionomic approach, integrating idiographic and nomothetic insights, with traditional multilevel modeling (MLM). Our methods included idiographic autoregressive integrative moving average models with an exogenous variable (i-ARIMAX), multivariate random-effects meta-analysis (RE-MA), deep Gaussian mixture modeling (DGMM), and multilevel vector autoregression modeling (Multilevel-VAR). The results showed that i-ARIMAX outperformed MLM in capturing within-person heterogeneity in the links between valued action and affect variables. Increases in values-based living were positively related to hedonic well-being but this effect showed a high degree of heterogeneity. A sub-group was identified, which we labeled the ‘Stoics,’ whose daily engagement in valued actions did not produce higher hedonic well-being (e.g., lower sadness or higher joy). Multilevel-VAR further revealed that for Stoics, stressful situations were linked to valued action, but not hedonic well-being. For Non-Stoics, valued action was less likely in stressful situations, but when valued action did occur it was associated with more joy and less sadness. The study offers initial evidence suggesting the superiority of an idionomic approach over a purely nomothetic one in capturing diverse pathways to clinically relevant outcomes. Idionomic methods may be useful or even necessary in personalizing psychological interventions, and thus may need to be considered by researchers and practitioners alike.