2020

Background. Intensive longitudinal (IL) designs provide the potential to study symptoms as they evolve in real-time within individuals. This has promising clinical implications, potentially allowing conclusions at the level of specific individuals. The current study aimed to establish the feasibility of IL designs, as indicated by self-rated burden and attrition, in the context of psychiatry. Additionally, we evaluated three core assumptions about the instruments (diary items) used in IL designs. These assumptions are: diary items (1) reflect experiences that change over time within individuals (indicated by item variability), (2) are interpreted consistently over time, and (3) correspond to retrospective assessments of psychopathology.

Methods. TRAILS TRANS-ID is an add-on IL study in the clinical cohort of the TRAILS study. Daily diaries on psychopathological symptoms for six consecutive months were completed by 134 at risk young adults (age 22.6 ± 0.6 years). At baseline, immediately after the diary period, and one year after the diary period, participants completed a diagnostic interview.

Results. Excellent compliance (88.5% of the diaries completed), low participant burden (M = 3.21; SD = 1.42; range 1-10), and low attrition (8.2%) supported the feasibility of six-month IL designs. Diary items differed in their variability over time. Evaluation of the consistency of diary item interpretations showed that within-individual variability in scores could not be attributed to changing interpretations over time. Further, daily symptom reports reasonably correlated with retrospective assessments (over a six month period) of psychopathology obtained with the diagnostic interview, suggesting that both measures might complement each other.

Conclusion. The current study is the first to show that IL designs over extensive periods (i.e., multiple months) in psychiatry are feasible, and meet three core assumptions to study change in psychopathology. This might allow for addressing novel and promising hypotheses in our field, and might substantially alter how we treat and study mental ill-health.

In 5 of the 6 large Dutch developmental cohorts investigated here, lower SES adolescents are underrepresented and higher SES adolescents overrepresented. With former studies clearly revealing differences between SES strata in adolescent social competence and behavioral control, this misrepresentation may contribute to an overestimation of normative adolescent competence. Using a raking procedure, we used national census statistics to weigh the cohorts to be more representative of the Dutch population. Contrary to our expectations, in all cohorts, little to no differences between SES strata were found in the two outcomes. Accordingly, no differences between weighted and unweighted mean scores were observed across all cohorts. Furthermore, no clear change in correlations between social competence and behavioral control was found. These findings are most probably explained by the fact that measures of SES in the samples were quite limited, and the low SES participants in the cohorts could not be considered as representative of the low SES groups in the general population. Developmental outcomes associated with SES may be affected by a raking procedure in other cohorts that have a sufficient number and sufficient variation of low SES adolescents.