Online Supplement B
CT Comparison Sample. To compare the results obtained in our sample of patients in
combined treatment to those in CT alone, we utilized data reported by Strunk et al. (2010a). That
sample was composed of 60 patients with a primary Axis I diagnosis of Major Depressive
Disorder who were randomly assigned to the CT condition of a two-site (University of
Pennsylvania and Vanderbilt University) clinical trial of CT, ADM, and placebo for moderate to
severe depression (see DeRubeis et al., 2005). To be eligible for that trial, patients had to meet
criteria for a current episode of depression (based on the SCID; First et al., 2001) and score 20 or
higher on the modified 17-item version of the Hamilton Rating Scale for Depression (Hamilton,
1960). The exclusionary factors were the same as those reported for our primary sample. Six
therapists provided CT (for more details, see DeRubeis et al., 2005). Among the 60 CT patients,
58% were women and ages ranged from 19 to 68 years (M = 40, SD = 12). Most patients were
Caucasian (78%); 12% were African American, and 10% were of other ethnicities.
Therapist Competence. To facilitate exploratory analyses involving therapist competence,
a principal investigator of both trials (Steven D. Hollon) provided global ratings of competence
for all cognitive therapists across both trials. These ratings were made based on his knowledge of
the therapists over the course of the trials. Cognitive therapists were rated using the overall
competence item from the Cognitive Therapy Scale (Young and Beck, 1988), with possible
responses ranging from a score of 0 (poor) to 6 (excellent). Ratings in the primary combined
treatment sample ranged from 0 to 5 (M = 3.7; SD = 1.4). Ratings for the CT alone sample
ranged from 2 to 5 (M = 4.3; SD = 1.2). The overall mean was 4.0 (SD = 1.4), which
corresponds to a rating of “good.”
Comparison of Process-Outcome Relations in Combined Treatment vs. CT alone
To compare the process-outcome relations across trials, we pooled the datasets and tested
interactions of each process measure with treatment (i.e., combined treatment vs. CT alone) as
predictors of subsequent session-to-session change. Recall that Strunk and colleagues reported
strong positive relations with greater symptom change for both the Cognitive Methods (r = -.44)
and Negotiating / Structuring (r = -.38) scales (but not Behavioral Methods / Homework, r =
.04). In our combined treatment sample, Behavioral Methods / Homework emerged as the most
robust predictor of session-to-session improvement (r = -.24), with Cognitive Methods and
Negotiating / Structuring failing to emerge as significant predictors (rs of .11 and .07,
respectively).
To test whether each of the process-outcome relations obtained differed across trials, we
tested interactions between treatment (i.e., study) and each process variable. In the pooled
dataset, there was a significant interaction between Cognitive Methods and treatment (r =-.21,
t(212) = -3.14, p = .002) and between Behavioral Methods / Homework and treatment (r = .17,
t(212) = 2.49, p = .01). There was a non-significant trend for the interaction between Negotiating
/ Structuring and treatment (r = .12 t(212) = -1.78, p = .08) and no evidence of an interaction of
alliance and treatment (r = -.00, t(212) = -.03, p = .98).
To address concerns about differences in inclusion criteria between the two clinical trials,
we conducted additional analyses restricting the sample to patients who either had chronic
depression or a prior depressive episode (inclusion criteria for the study of combined treatment)
and to patients with intake HRSD scores of 20 or greater (inclusion criteria for the study of CT
alone). In this restricted sample of the combined condition (n = 112) and CT alone (n = 52), we
examined the two significant interactions identified in the full sample. The interaction between
Cognitive Methods and treatment remained significant (r = .23, t(154) = -3.00, p = .003) while
the interaction between Behavioral Methods / Homework and treatment was reduced to the level
of a non-significant trend (r = .15, t(154) = 1.89, p = .06).
Exploratory Analysis of Therapist Competence
We were surprised that the strong association of Cognitive Methods and subsequent
symptom change identified by Strunk and colleagues (2010a) in CT alone was not evident in our
combined treatment sample. While the presence or absence of ADM might account for this
difference, we also suspected that variability in therapist competence might help to explain the
result. To explore this possibility, we examined the interaction of Cognitive Methods and ratings
of therapist competence in predicting subsequent session-to-session symptom change using data
from both the CT alone and the combined treatment samples. This interaction was significant (r
= -.16, t(212) = -2.39, p = .02). To explore the nature of this interaction, we examined the
relation of Cognitive Methods and subsequent symptom change separately for the 12 therapists
rated at or above the median competence rating of “good” and the eight therapists rated below
this level. Among the therapists rated as more competent, Cognitive Methods predicted
significantly greater symptom change (r = -.19, t(134) = -2.25, p = .03); among the therapists
rated as less competent, Cognitive Methods was associated with non-significantly less robust
symptom change (r = .15, t(78) = 1.31, p = .20). To determine whether the interaction of
competence and Cognitive Methods accounted for the interaction of Cognitive Methods and
treatment (CT alone vs. combined) noted earlier, we then tested a model with both interactions.
In this model, the interaction of Cognitive Methods and treatment remained significant (r = -.17,
t(211) = -2.57, p = .01). Thus, while we did find evidence for the interaction of competence and
Cognitive Methods, this interaction did not appear to account for the difference in the relation of
Cognitive Methods and subsequent symptom change observed across the CT alone and
combined treatment samples. There was no evidence of an interaction between therapist
competence ratings and Behavioral Methods/ Homework (r = .10, t(212) = 1.40, p =.16).
Conclusions
Across the CT alone and combined samples, we found significant differences in the
relation of both Cognitive Methods and Behavioral Methods / Homework to session-to-session
symptom change. A similar pattern emerged in a sample restricted to those patients who would
have been eligible for both trials, though the association between Behavioral Methods /
Homework and symptom change was reduced to a non-significant trend. Our analyses of
competence identified a significant interaction with Cognitive Methods. These analyses
suggested that Cognitive Methods were more predictive of symptom change among those
therapists judged to be more competent, but not predictive of improvement among therapists
judged as less competent. However, because the competence by Cognitive Methods interaction
did not fully explain the treatment by Cognitive Methods interaction, future research utilizing
more fine-grained assessments of competence may be important to shed further light on this
issue.
Two important limitations bear consideration. First, it is important to recognize that the
ratings of competence used in these analyses were made by an expert who was not blind to
patient outcomes. Nonetheless, because we failed to find a relation between competence ratings
and session-to-session symptom change (r = .05, t(216) = .80, p = .4), it is unlikely that any
interaction of competence and other process measures would be accounted for by bias in
competence ratings related to knowledge of patients’ outcomes. Secondly, the process ratings
provided for the two studies utilized in our pooled sample were generated by different groups of
raters. As such, we cannot rule out the possibility that rater-related measurement error may have
contributed to differences between process-outcome relations across samples. Nevertheless, we
have difficulty imagining a reason why such error would result in the pattern of results obtained.
References (Not Included in Manuscript)
Hamilton, M. (1960). A rating scale for depression. Journal of Neurological Neurosurgical
Psychiatry, 23, 56-62.
Young, J. E., & Beck , A. T. (1988). Revision of the Cognitive Therapy Scale. Unpublished
manuscript. University of Pennsylvania, Philadelphia, PA.