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Psychological group treatments, such as behavioral or cognitive-behavioral therapy, are generally effective interventions for social phobia. However, a substantial number of individuals discontinue these treatments prematurely. Participant attrition can threaten the validity of treatment outcome studies if attrition during therapy does not occur randomly. In order to examine this issue, we studied 133 individuals with a principal diagnosis of social phobia who initiated a 12-week behavioral or cognitive-behavioral group treatment for social phobia. Thirty-four participants discontinued therapy prematurely. These dropouts were compared to treatment completers in demographic characteristics, Axis I and II psychopathology, and their attitude toward treatment. The results only showed a small difference between treatment completers and dropouts in their attitude toward treatment: dropouts rated the treatment rationale as less logical than completers at the beginning of treatment. No other differences between dropouts and completers were observed. Therefore, dropouts are unlikely to present a serious threat to the external validity of treatment outcome studies for social phobia.

With a lifetime prevalence rate of 13.3%, social phobia (social anxiety disorder) is one of the three most common mental disorders in the general population (Kessler et al., 1994). The disorder typically follows a chronic course and results in substantial impairments in vocational and social functioning. Exposure and cognitive-behavioral therapies are efficacious forms of treatment for this condition, as suggested by various meta-analyses (Feske & Chambless, 1995; Gould, Buckminster, Pollack, Otto, & Yap, 1997; Taylor, 1996). These treatments are often administered in group format. Similar to other anxiety disorders, a high proportion of individuals with social phobia discontinues such treatments prematurely (i.e., drop out). For example, a recent randomized controlled trial that included cognitive-behavioral therapy for generalized social phobia found that 23% (out of 295) dropped out of treatment (Davidson et al., 2004). Similarly, Heimberg et al. reported dropout rates between 20% (Heimberg et al., 1990) and 22% (Heimberg et al., 1998). Patients in these studies were defined as dropouts if they missed more than 3 out of 12 group treatment sessions. Dropouts can cause a significant threat to the validity of a study if participants who drop out differ systematically across clinically relevant variables from those who complete treatment (Little & Rubin, 1989). This discussion has gained momentum again more recently in the field of clinical psychology. For example, some authors have questioned the external validity of findings from randomized controlled trials and the practical value of empirically supported treatments (Westen, Novotny, & Thompson-Brenner, 2004). One of the common arguments against the external validity of empirically supported treatments is that randomized controlled trials lead to positively biased findings because of selection effects that are caused by exclusion criteria and treatment attrition. Therefore, it is critically important to investigate the characteristics of individuals who discontinue a treatment trial prematurely.1 However, despite the methodological importance of this issue, surprisingly few studies have systematically examined dropouts in clinical research (for reviews see Baekeland & Lundwall, 1975; Wierzbicki & Pekarik, 1993). Studies examining the issue of attrition during the course of psychotherapy vary considerably on the variables investigated as possible indicators of treatment dropout. These variables have generally fallen into one or more of the following categories: (1) demographic variables; (2) clinical variables, and (3) patient attitude toward treatment (Carter, Turovsky, Sbrocco, Meadows, & Barlow, 1995; Dreessen, Arntz, Luttles, & Sallaerts, 1994; Grilo et al., 1998; Hansen, Hoogduin, Schaap, & deHaan, 1992; Oei & Kazmierczak, 1997; Persons, Burns, & Perloff, 1988; Rabin, Kaslow, & Rehm, 1985; Turner, Beidel, Wolff, Spaulding, & Jacob, 1996). The most recent dropout analysis was conducted by Keijsers, Kampman, and Hoogduin (2001). The authors compared 32 dropouts from cognitive-behavior therapy for panic disorder consisting of 15 individual sessions with 129 treatment completers in their levels of education, treatment motivation, personality psychopathology, and symptom severity. This study found a significant but small difference in the level of treatment motivation, with dropouts demonstrating less motivation than treatment completers. The only study investigating dropout from treatment for social phobia was conducted by Turner et al. (1996). This study recruited 84 individuals for a study comparing exposure therapy, atenolol, and pill placebo for social phobia. Thirteen of the 84 eligible participants refused to participate in the study, and nine individuals dropped out during the course of the 12-week treatment program (five from psychotherapy, three from atenolol treatment, and one from placebo). The authors found no differences between the dropouts and the completers in any of the demographic variables (age, gender, marital status, and education), or clinical features (comorbid Axis I diagnoses, social phobia subtype, and clinical severity rating of the social phobia diagnosis). However, dropouts had significantly lower pretreatment scores in the trait subscale of the State-Trait Anxiety Inventory (STAI-trait, Spielberger, Gorsuch, & Lushene, 1970). As noticed by the authors, these results were limited by the relatively small sample size and the omission of personality variables considered for the treatment dropout analysis. Given the high prevalence rate of social phobia, the documented efficacy of cognitive-behavioral group therapy, and the important implications of non-random attrition from therapy, it is important to examine dropouts and its correlates from treatment outcome studies of social phobia. Therefore, the goal of the present study is to examine the variables associated with dropout from these two treatments. To our knowledge, this study represents the most comprehensive examination of demographic variables, clinical variables, and patient attitude as correlates of dropout from cognitive-behavioral group therapy for social phobia to date.

A repeated measure ANOVA was conducted with the two treatment groups as the between factor (Group), the pre–post test as the within factor (Time), and the SPAI social phobia score as the dependent variable. The analyses revealed only a significant Time effect, F(1, 91) = 95.71, p < .0001, but no significant Group by Time interaction effect, F(1, 91) = 0.09, p > .7. Individuals from the behavioral and CBGT group showed a similar mean score in the SPAI social phobia scale at pre-test (M: 122.67, S.D.: 3.68 vs. M: 123.91, S.D.: 5.08, respectively) and post-test (M: 96.74, S.D.: 4.20 vs. M: 99.48, S.D.: 5.80, respectively). More detailed results about the efficacy of these two interventions were reported elsewhere ( Hofmann, 2004 and Hofmann et al., 2004). The two study treatments did not differ in rate of patient dropout (EGT: n = 22 or 20.69%; CBGT: n = 12 or 29.33%), χ2(1) = 0.66 (with continuity correction), p > .4. Because the treatments did not differ in their efficacy and dropout rates, the following analyses will combine the dropouts from the two treatments and compare them to the combined sample of treatment completers. 3