Ecological momentary assessment (EMA; Stone & Shiffman, 1994) was used to characterize and quantify a dynamic process--affective instability in borderline personality disorder (BPD). Sixty outpatients (34 with BPD and affective instability; 26 with current depressive disorder but not with BPD or affective instability) carried electronic diaries for approximately 1 month and were randomly prompted to rate their mood state up to 6 times a day. Results indicated that BPD patients (a) did not report significantly different mean levels of positive or negative affect; (b) displayed significantly more variability over time in their positive and negative affect scores; (c) demonstrated significantly more instability on successive scores (i.e., large changes) for hostility, fear, and sadness than did patients with depressive disorders; and (d) were more likely to report extreme changes across successive occasions (>or=90th percentile of change scores across participants) for hostility scores. Results illustrate different analytic approaches to quantifying variability and instability of affect based on intensive longitudinal data. Further, results suggest the promise of electronic diaries for collecting data from individuals in their natural environment for purposes of clinical research and assessment.
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