Two main sources of confusion dominate research on trait-like (between-patients) and state-like (within-patient) effects in psychotherapy. The first is that being higher than another person on a given construct (between-individuals differences) has the same statistical and clinical implications as showing increases from one time point to the next on that construct (within-individual changes). However, research shows that it is a mistake to mix together the two effects. The second is overlooking the interplay between trait-like (between-individuals) differences and state-like (within-individual) changes in the same construct, although such interactive effects between the two may reveal critical information for guiding clinical decision-making. Two types of such interplays are briefly discussed, as well as their distinct clinical implications. The first refers to a compensatory effect according to which those with the lowest trait-like levels on a mechanism of change are the ones benefiting most from state-like improvements in that mechanism, so that where you start from need not have a deterministic effect if you are going in a promising new direction; on the contrary, it may point to the most critical state-like change required to return to healthy homeostasis. The second refers to a complementary effect, according to which treatment should focus on identifying individuals' trait-like strengths and capitalizing on them in treatment planning to drive state-like improvement. Differentiating between the two types of interplay is critical for basing treatment planning (determining the required state-like changes) on the individual's case conceptualization (trait-like characteristics). (PsycInfo Database Record (c) 2022 APA, all rights reserved).
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