Extended validation of the Relational Depth Frequency Scale: Retest reliability, divergent validity, criterion validity with psychotherapy satisfaction, and measurement invariance in UK- and US-stratified samples.

Background: The Relational Depth Frequency Scale (RDFS) assesses moments of profound connection in psychotherapy, associated with therapeutic benefit. To date, the RDFS has not been tested for its retest reliability, divergent and criterion validity, and measurement invariance, nor has it been tested in stratified samples of psychotherapy patients.

Methods: Two stratified online samples of United Kingdom (n = 514) and United States (n = 402) psychotherapy patients filled out the RDFS, the Brief Social Desirability Scale (BSDS); and the Satisfaction with Therapy and Therapist Scale-revised (STTS-R). Two subsamples of patients (United Kingdom: n = 50 and United States: n = 203) filled out the RDFS again after 1 month.

Results: Reliability for the six-item RDFS were excellent in United Kingdom and United States samples (Cronbach's α = 0.91 and 0.92; retest r = 0.73 and r = 0.76). Divergent (r = 0.10 and r = 0.12) and criterion validity (r = 0.69; and r = 0.70) were good. Full scalar invariance was established across countries, genders, and time.

Conclusion: This contributes important evidence to the validity of the RDFS. Future research should assess predictive validity against psychotherapy outcomes and replicate these analyses in diverse samples.