A meta-analysis of dropout rates in acceptance and commitment therapy

Behaviour Research and Therapy 104 (2018) 14–33

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Behaviour Research and Therapy journal homepage: www.elsevier.com/locate/brat

A meta-analysis of dropout rates in acceptance and commitment therapy

T

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Clarissa W. Ong , Eric B. Lee, Michael P. Twohig Department of Psychology, Utah State University, 2810 Old Main Hill, Logan, UT 84322, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Meta-analysis Dropout Acceptance and commitment therapy

Many psychotherapies, including cognitive behavioral therapy and acceptance and commitment therapy (ACT), have been found to be effective interventions for a range of psychological and behavioral health concerns. Another aspect of treatment utility to consider is dropout, as interventions only work if clients are engaged in them. To date, no research has used meta-analytic methods to examine dropout in ACT. Thus, the objectives of the present meta-analysis were to (1) determine the aggregate dropout rate for ACT in randomized controlled trials, (2) compare dropout rates in ACT to those in other psychotherapies, and (3) identify potential moderators of dropout in ACT. Our literature search yielded 68 studies, representing 4,729 participants. The weighted mean dropout rates in ACT exclusive conditions and ACT inclusive conditions (i.e., those that included an ACT intervention) were 15.8% (95% CI: 11.9%, 20.1%) and 16.0% (95% CI: 12.5%, 19.8%), respectively. ACT dropout rates were not significantly different from those of established psychological treatments. In addition, dropout rates did not vary by client characteristics or study methodological quality. However, master's-level clinicians/ therapists (weighted mean = 29.9%, CI: 17.6%, 43.8%) were associated with higher dropout than psychologists (weighted mean = 12.4%, 95% CI: 6.7%, 19.4%). More research on manipulable, process variables that influence dropout is needed.

1. Introduction Treatment dropout rates for psychotherapy have been examined in a number of reviews. An early systematic review of psychotherapy dropout rates across 125 studies published before 1990 concluded that 46.9% of participants dropped out of treatment prematurely (Wierzbicki & Pekarik, 1993). A more recent review found an improved dropout rate of 19.7% across 669 studies published from 1990 to 2010 (Swift & Greenberg, 2012). Dropout rates for the different types of treatment were: 17.3% (supportive therapy), 18.4% (cognitive behavioral therapy; CBT), 19.1% (integrative), 19.2% (solution-focused), and 20.0% (psychodynamic). However, no significant differences in dropout rates were found among modalities. Another recent review of 115 CBT clinical trials found dropout rates of 15.9% before the start of treatment and 26.2% during treatment (Fernandez, Salem, Swift, & Ramtahal, 2015). Although informative, these meta-analytic reviews have not specifically examined dropout rates in modern forms of CBT. One such unexamined treatment modality is acceptance and commitment therapy (ACT; S. C. Hayes, Strosahl, & Wilson, 1999), a type of cognitive behavioral therapy that emphasizes acceptance, mindfulness, and valued action. The theorized mechanism of change in ACT is psychological flexibility, which can be defined as the ability to fully contact the

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present moment regardless of internal experiences that show up, while engaging in valued behavior (S. C. Hayes, Luoma, Bond, Masuda, & Lillis, 2006). ACT aims to improve psychological flexibility through six processes or skills, with the ultimate aim of increasing effective or meaningful action. The ACT processes include: acceptance (willingness to experience internal events), defusion (deliteralizing language that can govern behavior), contact with the present moment (grounding the self in the here and now), self-as-context (recognizing the self as a temporary vessel for internal events), values (self-chosen domains of living that provide meaning and purpose), and committed action (commitment to and engagement in valued behavior). A growing body of research has shown ACT to be an effective treatment across a broad range of problem areas that include: anxiety (Swain, Hancock, Hainsworth, & Bowman, 2013), chronic pain (Hann & McCracken, 2014), depression (Zettle, 2015), obsessive-compulsive spectrum disorders (Bluett, Homan, Morrison, Levin, & Twohig, 2014), and substance use (Lee, An, Levin, & Twohig, 2015). However, little is known about the overall acceptability of ACT and how it compares to that of other empirically supported treatments. As ACT becomes more established and popular among treatment providers, it is increasingly necessary to evaluate dropout rates in ACT. ACT emphasizes a willingness to experience thoughts, emotions, and bodily sensations, eschewing more traditional methods of evaluating

Corresponding author. E-mail address: [email protected] (C.W. Ong).

https://doi.org/10.1016/j.brat.2018.02.004 Received 21 August 2017; Received in revised form 2 January 2018; Accepted 13 February 2018 Available online 16 February 2018 0005-7967/ © 2018 Elsevier Ltd. All rights reserved.

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from an existing study, used a treatment that did not match the identified problem behavior (e.g., targeting shame in individuals with substance use), or that did not provide sufficient information on dropout rates were excluded from this review.

and attempting to change, remove, or control these experiences (e.g., cognitive restructuring). These strategies are used in other psychotherapies, such as CBT. The theory, philosophy, and methodology of ACT may be better suited to some individuals, whereas others may more readily engage in a traditional CBT approach. For example, a recent analysis of two randomized controlled trials identified moderators that differentiated between participants who were more likely to continue treatment for anxiety using either a traditional CBT or ACT approach (Niles, Wolitzky-Taylor, Arch, & Craske, 2017). The researchers found that those who perceived a high level of control of their anxiety, were taking medication for anxiety, were more religious, and were more avoidant of physiological arousal symptoms were more likely to drop out of ACT than CBT. On the other hand, individuals were more likely to drop out of CBT than ACT when they did not have these traits. A better understanding of predictors of dropout in ACT could be used to individualize treatment recommendations among the many empirically supported therapies, as well as identify variables that enhance treatment retention in ACT, consequently, bolstering treatment effectiveness. The overarching objective of the current meta-analysis was to examine dropout in ACT, as one of the key metrics of treatment utility. As such, the specific goals of our study were to: (1) systematically and statistically review current data on dropout rates in ACT across a broad range of psychological and behavioral health problems, (2) compare dropout rates in ACT to those in other psychological interventions, and (3) identify potential moderating factors that contribute to dropout in ACT, including client characteristics and therapy variables.

2.3. Risk of bias in individual studies To increase generalizability of present findings, we did not exclude studies based on methodological quality, provided that they met our eligibility criteria. We note that heterogeneity in reported dropout rates may be partly attributed to methodological quality, which we examined as a moderator. However, the variance introduced by study quality may also provide a more accurate representation of psychological interventions administered across different settings.

2.4. Coding Methodological quality. Given our broad inclusion criteria, each study was coded for methodological quality by two independent raters using the Psychotherapy Outcome Study Methodology Rating Scale developed by Öst (2008), which has been used in previous meta-analyses (A-Tjak et al., 2015; Öst, 2014). The scale assesses outcome studies in the areas of: clarity of sample description, disorder severity/ chronicity, sample representativeness, diagnostic reliability, specificity of outcome measures, psychometric quality of outcome measures, use of blind evaluators, assessor training, condition assignment, design (strength of comparison conditions), power analysis, assessment points, quality and replicability of intervention, number of therapists, therapist training/experience, checks for treatment adherence, checks for therapist competence, control of concomitant treatments, handling of attrition, statistical analyses and presentation of findings, clinical significance, and equality of therapy hours across conditions. Each area is rated from 0 (poor) to 2 (good), and verbal descriptions of each numerical score are provided in the scale. The intraclass correlation coefficient for total score between both raters was .99 (95% CI: .99–1.00), indicating excellent interrater reliability. Descriptive information. Data on participant and treatment characteristics, as well as dropout rates were extracted from each article. For analyses, we defined dropout as attrition following the start of therapy (i.e., after attending at least one session of intervention). Trained research assistants coded all studies, and 85% of articles were recoded by a second coder for accuracy. Discrepancies in coding were resolved by either the first or second author. Samples were coded by age group (adult, child/adolescent) and diagnosis (psychological, physical, behavioral health, mixed). Psychological conditions included presentations such as anxiety, depression, and eating disorders; physical conditions included chronic pain and fibromyalgia; and behavioral health conditions included substance use and obesity. Study conditions were categorized into treatment type (ACT [ACT exclusive], ACT+ [ACT plus another intervention or ACT inclusive], CBT, cognitive therapy [CT], behavior therapy [BT], active control, inactive control), therapy format (individual, group, mixed), mode of delivery (in-person, telehealth), and therapist experience (Ph.D./psychologist, M.D./physician, Master's level clinician/therapist, graduate student, no therapist, multidisciplinary team). Active control conditions included treatment as usual, whereas inactive control referred to waitlist conditions. When conditions used therapists with varying levels of experience within the same domain (e.g., psychologists and psychology graduate students), they were assigned to the category with less experience (i.e., graduate students) to err on the conservative side.

2. Method This meta-analysis was conducted in accordance with the PRISMA guidelines (Moher, Liberati, Tetzlaff, Altman, & The PRISMA Group, 2009). 2.1. Literature search Systematic literature searches were conducted on PsycINFO and PubMed in August 2017, using the keywords: “acceptance and commitment therapy” AND “randomized controlled trial OR RCT OR random*.” Search results were restricted to peer-reviewed journal articles published in English. We also identified articles from a list of ACT randomized controlled trials on the Association for Contextual Behavioral Science website, which was updated in March 2017 (https://contextualscience.org/ACT_Randomized_Controlled_Trials). After the removal of duplicate articles, abstracts were screened by the first and second authors. Full-length articles of abstracts that appeared to meet the study selection criteria were retrieved. The articles were then reviewed for eligibility. Any ambiguity regarding study eligibility was settled via discussion between the first two authors; a consensus was required for inclusion in the meta-analysis. 2.2. Selection criteria To be included in the present meta-analysis, studies had to meet the following criteria: (a) random assignment to treatment condition; (b) inclusion of at least one comparison condition (e.g., waitlist, treatmentas-usual); (c) participants with a psychological diagnosis, physical diagnosis, or behavioral health problem (i.e., clinical sample); (d) comprehensive ACT protocol (i.e., covered all six ACT processes); (e) faceto-face therapy; and (f) English-language publication. We included various modalities of therapy, including individual, group, and telehealth formats, as well as participants belonging to all age groups to increase generalizability of our findings. Studies that reanalyzed data

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Identification

C.W. Ong et al.

Records identified through database searching (n = 455)

Additional records identified through ACBS list of ACT RCTs (n = 182)

Screening

Records after duplicates removed (n = 342)

Included

Eligibility

Records screened (n = 342)

Full-text articles assessed for eligibility (n = 88)

Studies included in quantitative synthesis (meta-analysis) (n = 68)

Records excluded (n = 265)

Full-text articles excluded (n = 20) Non-random assignment (1) Non-clinical sample (4) Treatment protocol did not cover all ACT processes or was inconsistent with ACT (2) Too few treatment sessions (4) Data reported elsewhere (3) Insufficient data on dropout (4) Description of study protocol (2)

Fig. 1. Flowchart of literature search process. ACT = acceptance and commitment therapy; ACBS = Association for Contextual Behavioral Science; RCT = randomized controlled trial.

were selected given heterogeneity in study findings as indicated by Cochran's Q test (Cochran, 1954) and the I2 statistic (Higgins & Thompson, 2002; Higgins, Thompson, Deeks, & Altman, 2003). A metaregression with study condition as the predictor and dropout rate as the dependent variable was used to determine if dropout differed by treatment type. A second metaregression tested the effects of client characteristics (i.e., age group and diagnosis) on dropout rate in ACT. A third metaregression was conducted with therapy format, therapist experience, session count, and session frequency as predictors to identify potential therapeutic moderators of dropout in ACT. A fourth metaregression examined the effect of study methodological quality on dropout in ACT. Finally, a funnel plot with trim and fill as well as the random effects version of the Egger's regression test for funnel plot asymmetry were used to evaluate publication bias in our data (Duval & Tweedie, 2000; Egger, Smith, Schneider, & Minder, 1997).

2.5. Analytic plan Statistical analyses were conducted using R version 3.4.0 (R Core Team., 2015), and the R packages: tidyverse (Wickham, 2017) and metafor (Viechtbauer, 2010). Proportion data were first transformed using the Freeman-Tukey (double arcsine) transformation. This transformation is recommended for proportion data because it produces more stable estimates of corresponding sampling variances for the sampling distribution of proportions close to 0 or 1 (Barendregt, Doi, Lee, Norman, & Vos, 2013; Freeman & Tukey, 1950). The transformed proportions and their corresponding sampling variances were used in all analyses, and were back-transformed—based on the equation derived by Miller (1978)—in cases of significance for ease of interpretation. Weighted means (with inverse-variance weights) for dropout rates were computed using random effects models. Random effects models

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Table 1 Characteristics of included studies.

Age group Adult Adolescent/child Diagnosis Psychological Physical Behavioral health Mixed Conditiona ACT ACT+ CBT CT BT Active control Inactive control Therapy formata Individual Group Mixed Modea In-person Telehealth Therapist experiencea Ph.D./Psychologist M.D./Physician Master's level clinician/therapist Graduate student No therapist Multidisciplinary team Session counta Session frequency (per week on average)a Methodological quality a

All studies (k = 68)

ACT exclusive conditions (n = 56)

ACT inclusive conditions (n = 71)

64 (94.1%) 4 (5.9%)

51 (91.1%) 5 (8.9%)

66 (93.0%) 5 (7.0%)

36 (52.9%) 21 (30.9%) 9 (13.2%) 2 (2.9%)

29 (51.8%) 19 (33.9%) 6 (10.7%) 2 (3.6%)

37 (52.1%) 23 (32.4%) 9 (12.7%) 2 (2.8%)

56 (38.4%) 15 (10.3%) 13 (8.9%) 2 (1.4%) 3 (2.1%) 33 (22.6%) 24 (16.4%)

56 (100%)

56 (78.9%) 15 (21.1%)

54 (50.9%) 39 (36.8%) 13 (12.3%)

30 (54.5%) 22 (40.0%) 3 (5.5%)

35 (51.5%) 26 (38.2%) 7 (10.3%)

104 (98.1%) 2 (1.9%)

54 (98.2%) 1 (1.8%)

68 (98.6%) 1 (1.4%)

17 (14.0%) 6 (5.0%) 18 (14.9%) 33 (27.3%) 30 (24.8%) 17 (14.0%) 11.5 (7.4) 1.2 (0.8) 20.8 (7.3)

11 (25.0%) 1 (2.3%) 8 (18.2%) 17 (38.6%) 0 (0%) 7 (15.9%) 11.3 (4.6) 1.0 (0.3)

13 (23.2%) 1 (1.8%) 10 (17.9%) 22 (39.3%) 0 (0%) 10 (17.9%) 11.6 (6.5) 1.1 (0.5)

Reflects condition-level data.

3. Results

15.8% (95% CI: 11.9%, 20.1%, range = 0–84.9%, Q[55] = 283.44 [p < .001], I2 = 78.75%, k = 56). When ACT inclusive conditions (i.e., all those that included an ACT intervention) were considered, the weighted mean dropout rate was 16.0% (95% CI: 12.5%, 19.8%, range = 0–84.9%, Q[70] = 365.26 [p < .001], I2 = 78.73%, k = 71). Figs. 2 and 3 are forest plots that represent condition-level dropout rates and corresponding 95% confidence intervals for the ACT exclusive and ACT inclusive (without ACT exclusive) conditions, respectively. To eliminate the confounding effects of an intervention other than ACT, the following results pertain to ACT exclusive conditions.

3.1. Study selection Fig. 1 summarizes the literature search process. A total of 637 articles were identified from initial searches, and 342 unique abstracts were reviewed for eligibility. Among those abstracts, 88 full-length articles were selected for more thorough review. Ultimately, 68 research articles, representing 4729 participants, were included in the present meta-analysis. 3.2. Descriptive information

3.4. Treatment condition comparison

Study characteristics are presented in Table 1. The majority of studies used adult samples, psychological conditions, individual therapy, and in-person format. Most ACT sessions were conducted by psychologists and graduate students (see Appendix for a detailed description of included studies).

The metaregression revealed no significant differences in dropout rates among conditions (ps > .05). The differences between ACT and CBT (weighted mean = 25.3%, 95% CI: 16.9%, 34.8%, p = .052), as well as between ACT and inactive control (weighted mean = 9.5%, 95% CI: 5.6%, 14.1%, p = .072) were notable, but not statistically significant (see Table 2). Specifically, ACT was associated with a lower dropout rate than CBT, and with a higher dropout rate than inactive control conditions.

3.3. Dropout rates in ACT The weighted mean dropout rate in ACT exclusive conditions was

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Fig. 2. Forest plot of dropout rates and corresponding 95% confidence intervals for acceptance and commitment therapy exclusive conditions.

(weighted mean = 27.2%, 95% CI: 9.2%, 49.8%) compared to that for psychological conditions (weighted mean = 16.4%, 95% CI: 11.0%, 22.5%, p = .098), with higher dropout observed for behavioral health problems. Therapy format, session count, and session frequency did not

3.5. Predictors of dropout Neither age group nor client diagnosis significantly predicted dropout rate in ACT (ps > .05; see Table 3). There was a nonsignificant difference between the dropout rate for behavioral health problems 18

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Fig. 3. Forest plot of dropout rates and corresponding 95% confidence internals for acceptance and commitment therapy inclusive conditions.

Table 2 Results from metaregression analysis of treatment conditions on dropout. Moderator (k) a

ACT exclusive (56) ACT inclusive (15) Cognitive behavioral therapy (13) Cognitive therapy (2) Behavior therapy (3) Active control (32) Inactive control (24)

Dropout rate

95% CI

z

p

15.8% 16.7% 25.3% 5.3% 17.7% 17.4% 9.5%

11.9%, 20.1% 9.1%, 25.8% 16.9%, 34.8% 0.9%, 11.8% 9.9%, 26.8% 12.7%, 22.6% 5.6%, 14.1%

0.28 1.94 −0.95 0.01 0.47 −1.80

.776 .052 .342 .993 .638 .072

Note. ACT = acceptance and commitment therapy; CI = confidence interval. a Reference level in metaregression.

Table 3 Results from metaregression analysis of client, therapy, and study moderators on dropout in ACT. Moderator (k) Age group Adulta (51) Adolescent/child (5) Diagnosis Psychologicala (29) Physical (19) Behavioral health (6) Mixed (2) Therapy format Individuala (30) Group (22) Mixed (3) Therapist experiencea Ph.D./Psychologista (11) M.D./Physician (1) Master's level clinician/therapist (8) Graduate student (17) Multidisciplinary team (7) Session count Session frequency (per week on average) Methodological quality

Dropout rate

95% CI

z

p

15.3% 21.4%

11.2%, 19.8% 11.2%, 33.6%

1.12

.264

16.4% 12.1% 27.2% 14.0%

11.0%, 22.5% 7.5%, 17.5% 9.2%, 49.8% 7.7%, 21.5%

−1.16 1.65 −0.26

.246 .098 .792

16.0% 12.8% 45.7%

11.2%, 21.4% 9.0%, 17.1% 7.2%, 87.4%

0.57 0.62

.569 .535

12.4% 26.7% 29.9% 15.6% 13.1% – – –

6.7%, 19.4% 6.8%, 52.3% 17.6%, 43.8% 11.0%, 20.7% 5.7%, 22.6% – – –

1.04 2.35 0.68 0.31 0.73 1.03 0.76

.296 .019 .498 .760 .464 .305 .447

Note. ACT = acceptance and commitment therapy; CI = confidence interval. a Reference level in metaregression.

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Fig. 4. Funnel plot with transformed proportions on the x-axis and their corresponding sampling variances on the y-axis, with missing studies filled in using the trim and fill method.

ACT and CBT (A-Tjak et al., 2015), determining treatment dropout provides another metric of the practical utility of ACT. We found that the dropout rate for ACT exclusive conditions was 15.8%, and the rate for ACT inclusive conditions was 16.0%. Both figures are close to those reported for other interventions, including adult psychotherapies (e.g., CBT, psychodynamic; 19.7%; Swift & Greenberg, 2012), CBT for mental health concerns (26.2%; Fernandez et al., 2015), individual psychotherapy for major depression (19.9%; Cooper & Conklin, 2015), and exposure and response prevention for obsessive-compulsive disorder (14.7%; Ong, Clyde, Bluett, Levin, & Twohig, 2016). The observed consistency between dropout rates in ACT, an acceptance- and mindfulness-based CBT, and other psychotherapies coheres with a previous meta-analysis that found no significant differences in dropout across treatment orientations (Swift & Greenberg, 2012). The heterogeneity in dropout rates indicated by Cochran's Q tests and I2 statistics suggested the presence of moderating variables. Yet, we only identified one significant moderator of dropout: therapist experience. Specifically, ACT administered by master's-level clinicians significantly predicted higher rates of dropout (26.4%) than when therapy was conducted by psychologists (12.6%). Findings on therapist experience as a moderator have been mixed, with two meta-analyses indicating no significant effect (Cooper & Conklin, 2015; Fernandez et al., 2015) and one reporting higher dropout rates when trainees (working toward a degree) provided the intervention (Swift & Greenberg, 2012). Of note, our analyses did not reveal a significant difference between psychologist and graduate student therapists. As graduate students are required to work under a licensed clinical psychologist, it is possible that the continued supervision maintained treatment integrity. However, replication of this result in other settings is warranted before strong inferences can be made. The overall lack of significant findings could have been due to lack of power, as a previous meta-analysis reported that diagnostic group and number of sessions significantly predicted dropout in CBT across 115 and 89 studies, respectively (Fernandez et al., 2015). Alternatively, factors that were not assessed consistently across studies, such as

significantly predict dropout rate in ACT (ps > .20; see Table 3). Relative to by psychologists (weighted mean = 12.4%, 95% CI: 6.7%, 19.4%), ACT administered by master's-level clinicians/therapists were associated with a higher dropout rate (weighted mean = 29.9%, 95% CI: 17.6%, 43.8%, p = .019). However, no differences were found between psychologists and graduate students (weighted mean = 15.6%, 95% CI: 11.0%, 20.7%, p = .50) or multidisciplinary teams (weighted mean = 13.1%, 95% CI: 5.7%, 22.6%, p = .76). Study methodological quality was also tested as a potential moderator of dropout in ACT conditions, given the heterogeneity of study design and quality included in this review. There was no significant effect of methodological quality on dropout in ACT (p = .45). 3.6. Publication bias Fig. 4 shows a funnel plot representing the distribution of transformed dropout rates in ACT exclusive conditions, with missing studies represented by blank circles using the trim and fill method (Duval & Tweedie, 2000). The transformed dropout rates are positively correlated with actual dropout rates, and so can be interpreted similarly. The funnel plot depicts slight asymmetry; conditions with lower transformed rates across the range of sampling variances tended to be overrepresented, implying that our sample may be biased in the direction of excluding studies with higher dropout rates. This visual observation was corroborated by results from the Egger's regression test, which indicated that the funnel plot was significantly asymmetrical (p < .05). These results suggest that the aggregate figures reported could be an underestimation of “true” dropout rate. 4. Discussion The purpose of the present study was to examine dropout rates in ACT, how they compare to those in other psychological interventions, as well as moderators of dropout in ACT. Given previous research that has demonstrated no differences in treatment effectiveness between

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moderation analyses in future meta-analyses. Furthermore, identifying manipulable aspects of treatment that reduce probability of dropout can guide the development of more acceptable interventions. Attending to such factors—beyond those that contribute to greater treatment efficacy—is important because the most effective interventions are rendered futile by premature termination.

therapeutic alliance—and that were hence excluded from our analyses—could have explained the variability in dropout (Cooper et al., 2016). Initial metrics like this are important to form a base for future researchers to continue to study similar issues. The range of diagnostic concerns represented in our sample reflects the flexibility of ACT treatment protocols used across studies, given that the same six processes were used to target a variety of clinical presentations. It is thus possible that the transdiagnostic nature of ACT, which leads to topographical differences in the presentation of therapy even within the same study, also contributed to the variability in dropout rate. For example, an established ACT protocol used in an RCT for OCD explicitly instructs therapists to introduce ACT processes in a flexible manner, based on clients’ presenting concerns (Twohig et al., 2010). A closer examination of how therapy is actually carried out (e.g., order in which processes are introduced, functionality of therapist response)—perhaps through viewing and coding therapy sessions—might elucidate the critical elements that lead to premature termination. It is worth briefly discussing the treatment condition and diagnostic group modifiers that were notable, yet not statistically significant. The higher dropout rate of CBT compared to ACT conditions (24.9% vs. 15.8%) was close to that reported by Fernandez et al. (2015) of 26.2%, thus, it is unlikely that the difference was due to therapist allegiance. Rather, it might be that the relative nascence of ACT as an intervention circumscribed its application to research labs focused on ACT, with clinicians well trained in ACT conducting the intervention. It is possible that dropout rates will change as the use of ACT proliferates. In addition, the lower dropout rate in inactive control conditions (9.5% vs. 15.8%) could be attributed to lower participant burden in terms of time and effort, given that participants were not required to attend any sessions at all. Finally, behavioral health problems tended to predict more dropout than psychological conditions (25.5% vs. 15.4%). This could be because substance use (e.g., cigarette smoking, opioid dependence) were coded as behavioral health problems. Interventions for substance use disorders are reported to be commonly associated with high dropout, with rates reaching as high as 90.4% (Brorson, Arnevik, Rand-Hendriksen, & Duckert, 2013; Kern-Godal, Arnevik, Walderhaug, & Ravndal, 2015); this pattern might have been reflected in our results. Still, inclusion of more studies and increased statistical power are needed to more accurately estimate the effects of these modifiers on dropout. Our findings suggest that, in addition to comparable effectiveness of ACT to established treatments (A-Tjak et al., 2015), ACT also shows comparable dropout rates. Individual studies have demonstrated high acceptability of ACT based on participant ratings (e.g., Johns et al., 2016; Twohig et al., 2010), and this meta-analysis corroborates preliminary indicators of client-reported acceptability using a behavioral metric. Still, the lack of information on moderators underscores the need for more systematic collection and reporting of data using standardized, psychometrically validated measures on process of change variables that have been shown to influence dropout, such as therapeutic alliance and use of behavioral strategies and homework (Cooper et al., 2016; Sharf, Primavera, & Diener, 2010), as well as other potentially relevant variables, such as client-reported treatment acceptability and treatment manual adherence. Doing so may facilitate

4.1. Limitations First, our exclusive focus on RCTs limits generalizability of our findings to other settings given that the nature of treatments provided in research studies are not representative of how they are conducted in clinical practice (e.g., time-limited course). Moreover, real-world settings may introduce more client and treatment variables that increase variability in dropout rates. Second, although we attempted to include moderating variables that may be relevant to dropout (e.g., perceived treatment acceptability, therapeutic alliance), insufficient data precluded such analyses. Furthermore, even when data were provided, they were not based on the same or standardized scales, making comparison of scores difficult. Third, we only examined dropout rates based on overall attrition due to limited availability of data in included studies. However, more comprehensive reporting on specific aspects of attrition (e.g., point in therapy at which participants drop out, reasons for dropout) may provide insight into differential factors that influence dropout. Fourth, the weighted aggregate dropout rate in ACT might not have been derived from a balanced sample of clinical trials, and it is possible that dropout rates are higher than reported. The “file drawer problem” is a well-documented source of publication bias that may undermine the accuracy of meta-analytic findings as significant findings are more likely to be published than nonsignificant findings (Rosenthal, 1979). It is plausible that such bias may apply to dropout rates as well, such that studies with higher dropout rates are less likely to be published. The presence of this effect was supported by results from the Egger's regression test as well as our funnel plot (see Fig. 4). Hence, current findings should be considered conservative estimates of actual dropout rates. Finally, authors used varying definitions of dropout, which necessarily imbues our metadata with inconsistency. In certain studies, only those who completed all intervention sessions were considered completers, whereas others considered participants who completed more than half of the prescribed course of treatment completers. Greater clarity in reporting (e.g., breakdown of treatment completion progress) would facilitate more accurate data extraction.

Funding source This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Statement of competing interests The authors have no competing interests to declare.

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19 2004 Dahl, Wilson, & Swedish public Nilsson health service employees experiencing daily pain or stress symptoms that were attributed to work 76 2004 Gifford et al. Self-identified nicotine-dependent adult smokers smoking 10 cigarettes or more per day for at least 12 months

36.3

80 Adults experiencing auditory hallucinations or delusions at the time of their admission to a state psychiatric hospital

2002 Bach & Hayes

22

40

43

89.5

59

39.4

Gender Mean age (% (years) female)

Sample description N

Year Authors

Qualitative Description of Included Studies.

Appendix

11.68

77

–

75

–

13.2

Ethnicity (% White)

SD age (years)

Nicotine replacement treatment

ACT

MTAU

Session length (minutes)

–

–

1, with individual consultation if nonadherent

2/week

1/week

50 (individual), 90 (group) 90

–

60

45–50 First within 72 h of agreeing to participate, second within 72 h of first, third held 3–5 days after second, and fourth within 72 h of discharge 1 or 2/week 40

Session frequency

4

3+

4

Session count

Psychologist and 14 graduate students Psychiatrist and 1 psychiatry resident

Psychologist or psychology intern CBT psychotherapist and registered nurse –

TAU

MTAU + ACT

Psychologist or psychology intern

Therapist experience

ACT + TAU

Treatment condition

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23

Adults with DSMIV diagnosis of TTM Adults with primary DSM-IV diagnosis of GAD

Adult women meeting five or more criteria for BPD Adults with medical diagnosis of epilepsy

11.5 7.8

36.7 38.9

7.8 11.2

11.74

50

33.59

62.5

–

87.1

96.4

–

–

9.8

100

87

9.98

Range = 23 to 64

51.65 48.2 55.1

71

31

34.6

40.675

33.32

42.2

76.19

89.28

51.85

100

28

27

22

Adults with DSM- 124 51 IV diagnosis of substance abuse or dependence and had relapsed to that substance during the last 30 days

21 Adults with 2008 Wicksell, Ahlqvist, Bring, medical diagnosis Melin & Olsson of whiplash associated disorders with pain duration lasting more than three months 2009 Petersen & Adults with DSM- 24 Zettle IV alcohol abuse or dependence and unipolar depressive disorder

2006 Woods, Wetterneck & Flessner 2008 Roemer, Orsillo & SaltersPedneault

2006 Lundgren, Dahl, Melin & Kies

2006 Gratz & Gunderson

2004 Hayes et al.

TAU

ACT

Graduate student Counselors

NA Psychologists and physician NA

Waitlist Exposure and acceptance therapy Waitlist

ACT + habit reversal training Waitlist Acceptance-based behavior therapy

Supportive therapy

Until discharge Until discharge

NA

NA 10

NA 16

4 individual, 2 group 4 individual, 2 group 10

NA

NA Psychologists and institution staff Psychologists and institution staff Master's-level therapist NA Graduate students

– 1/week

– 14

Based on availability

2/week

NA 10 over 8 weeks NA

1/week, then 1/2 weeks NA 1/week

–

–

NA

60 (individual), 90 (group)

3/week

32 individual, 16 group

60

30

NA

NA 90 (4 sessions), 60 (12 sessions) NA 60

90 (individual), 180 (group) 90 (individual), 180 (group) –

NA

– 90

60 (individual), 90 (group)

3/week

32 individual, 16 group

Therapists with at least 2 years of experience in the treatment of substance abuse and in use of behavior therapy Therapist with at least 5 years' experience in the treatment of substance abuse, sponsor – –

ACT + epilepsy control

Methadone maintenance Emotion regulation group intervention + TAU TAU

Intensive Twelve-Step Facilitation + methadone maintenance

ACT + methadone maintenance

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

Adults who smoke 15 + cigarettes a day for at least twelve months

24

2011 Westin et al.

Adults with primary diagnosis of tinnitus

64

46.9

56

71.05 81

303 58.7

38 2011 Hayes, Boyd, & Adolescents Sewell experiencing moderate to severe depressive symptoms

2011 Gifford et al.

2010 Twohig et al.

Adults (16 and up) 104 40 with DSM-IV criteria methamphetamine abuse or dependence Adults with DSM- 79 61 IV diagnosis of OCD

78.13

2010 Smout et al.

32

Adolescents with pain duration of more than 3 months

2009 Wicksell, Melin, Lekander & Olsson

50.9

15.49

14.9 14.61

45.99

37

30.9

14.8

12.9

1.35

2.55 3.1

12.5

15.5

6.5

2.4

Waitlist

Multidisciplinary treatment approach

ACT

Psychologists and physician

NA

Psychiatrist, psychologist, physiotherapist, and physician – ACT Psychologist and master's-level clinician CBT Psychologist and master's-level clinician 88.6 ACT Graduate students Progressive relaxation Graduate training students 86.8 ACT and medication Master's-level therapist and graduate students Medication NA Clinicians 37 born in ACT working under Australia, 1 psychiatrists, from an including indigenous psychologists, background social workers, psychiatric nurses, and occupational therapists. CBT Clinicians working under psychiatrists, including psychologists, social workers, psychiatric nurses, and occupational therapists. – ACT Psychologists and graduate students Tinnitus retraining therapy Physician

–

2/week

NA –

10 individual, 10 group NA –

NA

1 with follow- 150 up NA NA

1

60 (session 2 was 75)

1/week

–

NA –

–

60

60

60

60

60 (10 sessions), 90 (2 sessions) Variable; 4060

10

–

1/week

8

–

1/week

1/week

12

8

1/week

1/week

7 to 56

12

1/week

12

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

25

2012 Mo'tamedi, Rezaiemaram, & Tavallaie

2012 Jensen et al.

2012 Folke, Parling & Melin

Adult females 43 diagnosed with fibromyalgia Female adults with 30 diagnosis of primary chronic (migraine and tension-type) headache according to the International Classification of Headache Disorders

45.6

34.18 37.87

100

7.39 8.74

6.4

9.46

10.55

31.93

43.24

11.7

8.63 10.97

12.5

6.5

37.93

33.57 34.54

100

Adults with DSM- 128 52.3 IV diagnosis of at least one anxiety disorder Adults with public 45 80 speaking anxiety who met DSM-IVTR criteria for nongeneralized SAD Adults with DSM- 35 88.2 IV diagnosis of unipolar depressive disorder

2012 Arch et al.

2012 England et al.

Adults who met ICD-10 criteria for a psychotic disorder

2011 White et al.

22.22

54.9

Adults with chronic 114 50.9 pain for at least 6 months

2011 Wetherell, Afari, Rutledge et al.

27

70.8

47.5

21

Adults at least 60 years old with a principal DSM-IV diagnosis of GAD

2011 Wetherell, Afari, Ayers et al.

100% Iranian

Waitlist ACT MTAU

NA 8 8

12

–

Psychologists and physician NA Psychiatrist Psychiatrist

–

– Control (access to public health services) ACT

NA 1/week 1/week

1/week

–

1/week

1/week

6

1 individual, 5 group

1/week

1/week

1/week

1/month –

6

12

12

10 –

1/week

1/week

8

8

1/week

1/week

12

12

Graduate students

Postdoctoral and master's-level clinicians Postdoctoral and master's-level clinicians Psychologists and graduate student Psychologist and graduate student Psychologist Psychiatrist and community psychiatric nurse or occupational therapist Graduate students Graduate students Graduate students Graduate students

ACT

Acceptance-based exposure Habituation-based exposure

CBT

ACT

ACT TAU

CBT

ACT

CBT

ACT

100

64.4

67.2

96.3

67.5

62.5

NA 90 –

90

60-90 (individual), 120–180 (group) –

120

120

60

60

60 –

90

90

60

60

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

26

2013 Steiner, Bogusch & Bigatti 2013 Wicksell et al.

Female adults with 40 fibromyalgia

100

45.1

48.63

58

68.5

100

36.55 32

64.52 100

31 Adult female inmates with substance use disorder 73 Adults with persistent pain for longer than 3 months Female adults with 28 fibromyalgia

32.66

52.83

32.92

34 34.94

65.4

45.69

6.6

12.96

12.8

11.58 6.2

9.07

– 12.53

12.24

12.81

8.7

39.4

–

79.3

97.3

65 –

53

62 70

80.2

62.3

88

1/week, 1/2 weeks

16

Waitlist

Psychologists and physician NA

NA

12

8 8

– – ACT Pain management education ACT

– Physicians

TAU

4

NA 16 NA

12

NA

1/week

1/week 1/week

3/week, then 1/week –

NA 1/week NA

1/week

1/week

1/week, 1/2 weeks

16

12

1/week, then 1/2 weeks

1/week, then 1/2 weeks

1/week

1/week

– –

–

1/week

30

30

24

24

5 5

–

12

Psychologists

Psychologist and psychiatrist NA – NA

Master's-level clinicians Master's-level therapists Psychologists and graduate students Psychologists and graduate students Postdoctoral fellows and graduate students Postdoctoral fellows and graduate students Psychologist and psychiatrist

Public mental health services Psychologists Psychologists

Clinicians

ACT + TAU

Mindfulness and acceptance-based group therapy Cognitive behavioral group therapy Waitlist ACT Waitlist

Applied relaxation

ACT

Standard behavioral treatment

Acceptance-based behavioral treatment

Drug counseling

ACT

77

10.7

TAU

ACT + TAU

40.3

–

ACT Relaxation training

9.33 9.02

43 (median) 36-45 (IQR) – 48.5 38-55 (IQR) (median)

35.6 34

137 54 49.06

81

2013 Villagrá Lanza & González Menéndez

Adults with a principal DSM-IV diagnosis of GAD

Adults with DSMIV-TR diagnosis of social anxiety disorder

2013 McCracken, Sato & Taylor

92.68 90.5 95

21 76.19 Adults diagnosed with MS with elevated symptoms of depression and/ or anxiety Adults with opioid 56 37.5 dependence undergoing detoxification Adults with a BMI 128 – between 27 and 40 kg/m2

Adults with four or 41 more DSM-IV criteria for BPD

2013 Kocovski, Fleming, Hawley, Huta, & Antony

2013 Hayes-Skelton, Roemer, & Orsillo

2013 Forman et al.

2012 Stotts et al.

2012 Morton, Snowdon, Gopold, & Guymer 2012 Nordin & Rorsman

NA

90

60 60

–

240

NA 90 NA

120

120

90 (4 sessions), 60 (12 sessions)

90 (4 sessions), 60 (12 sessions)

75

75

50

50

– –

–

120

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

27

2014 Tamannaeifar, Gharraee, Birashk, & Habibi 2014 Vakili, Gharraee, Habibi, Lavasani & Rasoolian

2014 Mojtabaie & Asghari

2014 GonzalezMenendez, Fernandez, Rodriguez, & Villagra 2014 Luciano et al.

28.37

45.98

100

25.2 24.7 25.7 26.96

44.4

6.83

4.3 4.2 4.4

5.71 –

48.28 –

100

100

5.94 5.87

7.5

7.91 7.53

6.76

12.35

13.1

48.88 47.77

33.59

33.28 32.83

43.46

67.21

100

20–35

36.17

0

66.7

156 96.2

Women with breast 30 cancer with depressive symptoms 20 Adults with a primary DSM-IV diagnosis of major depressive disorder Adults with DSM- 32 IV-TR diagnosis of OCD

Adults with diagnosis of fibromyalgia

Adult women with 30 diagnosis of chronic tension headaches and migraines 37 Adult female inmates diagnosed with current abuse or dependence

2014 GharaeiArdkani et al.

2014 Craske et al.

2014 Clarke, Kingston, James, Bolderson, & Remington

Adults with ICD-10 24 (DCR) diagnosis of schizophrenia 61 Adults who had received at least one previous 8session course of psychotherapy and had been rereferred Adults with DSM- 87 IV diagnosis of social phobia, generalized type

2014 Chowdhary & Jahan

51

Adults with DSMIV diagnosis of GAD

2014 Avdagic, Morrissey, & Boschen

100% Iranian

100% Iranian

–

–

50.57

–

–

Psychologist 8 Psychiatrist – Psychologist and 8 psychiatrist

12

– CT ACT SSRIs ACT + SSRIs

12

– ACT

8 NA

16

16

NA 8 NA

12

– – – –

– – –

–

NA 45–60 NA

150 NA

90

90

NA 60 NA

60

60

120

120

60 –

120

120

2/week

2/week

NA – NA

– NA

1/week

1/week

NA 1/week NA

1/week

1/week

1/week

16

12

1/week

2/week –

1/week

1/week

16

8–10 –

6

6

NA 8 NA

Psychologist NA

Graduate student Psychologist

Psychologists and graduate students Psychologists and graduate students NA – NA

Clinical psychologists Clinical psychologist, nurse specialist, counselor

Psychologist and graduate student Psychologist and graduate student – –

NA – NA

ACT Recommended pharmacological treatment Waitlist ACT Waitlist

CBT

ACT

Waitlist ACT Control (no intervention)

CBT

ACT

CBT

ACT

ACT + TAU TAU

CBT

ACT

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

28

30

88

135 84.57

73.3

69.8

43

60

50

54

13

–

–

–

27.5

–

–

–

ACT

–

ACT TAU (Narcotics Anonymous) TAU (methadone replacement)

ACT + selective optimization compensation strategies Minimal support group

Psychoeducation ACT Waitlist

CBT

Applied relaxation ACT

ACT

Waitlist

Acceptance-based depression and psychosis therapy + MTAU Enhanced assessment and monitoring + MTAU

ACT Waitlist

15% Hispanic

–

10

4.42

13.54

11.4

8.9

17

8.75

82.26

39.97

61.82

40.3

36.9

36.25

126 70.63

101 78.6 Adults at least 65 years old with chronic musculoskeletal pain for at least six months 60 – Adult opiate dependents admitted to a drug rehab center

Adults diagnosed with chronic pain

2015 Nasiri & KazemiZahrani 2016 AlonsoFernandez, Lopez-Lopez, Losada, Gonzalez, & Wetherell 2016 Azkhosh, Farhoudianm, Saadati, Shoaee, & Lashani

2015 Losada et al.

2015 Kemani et al.

2015 Gloster et al.

Adults who met diagnostic criteria for severe health anxiety Adults with DSMIV diagnosis of major depressive disorder, severe with psychotic features, or schizoaffective disorder, depressive type Adults diagnosed with panic disorder or agoraphobia who have had one or more previous courses of psychological and/ or pharmacological treatment Adults with unspecific pain for at least six months Adult dementia family caregivers with significant depressive symptoms

2015 Eilenberg, Frostholm, Schroder, Jensen, & Fink 2015 Gaudiano et al.

1 8 NA 9

1

–

–

12 – –

Ph.D. – –

8

12 8

Psychologist and physician Psychologist Psychologists and master'slevel clinicians Psychologists and master'slevel clinicians – – NA

12

NA

8

–

1/week –

NA

1/week

NA 1/week NA

1/week

1/week 1/week

1/week

NA

2/week

–

–

Community clinician

Graduate students NA

1/week

– NA

16

10 NA

Psychologist

Psychologists NA

–

90 –

120

120

120 90 NA

90

90 90

90

NA

90–120

–

–

180 NA

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

Adults with problematic pornography use

Range = 18 to 47

29

2016 Yazdanbakhsh, Kaboudi, Roghanchi, Dehghan, & Nooripour 2017 Clarke, Poulis, Moreton, Walsh, & Lincoln

2016 Parling, Cernvall, Ramklint, Holmgren, & Ghaderi

Adults diagnosed with knee or hip osteoarthritis

31

Adolescents with 30 chronic pain 43 Adults with a diagnosis of AN or EDNOS who had received 9–12 weeks of daycare at a specialist eating disorder unit Adult women with 30 multiple sclerosis

2016 Kanstrup et al.

66.5

9

–

–

100

70.97

7.15

25.7

1.6

8

2.76

96.77

–

–

–

75

78

ACT TAU

ACT Waitlist

TAU

ACT ACT ACT + TAU

Present-centered therapy

Waitlist ACT

CBT

Ph.D. –

– NA

– Psychologists and graduate students Psychologists and graduate students NA Psychologists and social workers Psychologists and social workers Psychologist Psychologist Psychologist and graduate students – TAU ACT

–

–

–

NA

Waitlist ACT

–

ACT

0.85

–

Psychologist and graduate students NA – NA

8.75

ACT

Waitlist ACT Waitlist

92

33

11.4

97.67

16

34.2

Adult veterans with 160 20 DSM-IV diagnosis of at least one anxiety or depressive disorder

2016 Lang et al.

80

11.2

100

53

19.06

36.25

0

18

29.3

126 70.5

0

28

193 58

Adults scoring 3 or higher on the South Oaks Gambling Screen 2016 Eilenberg, Fink, Adults who met Jensen, Reif, & diagnostic criteria Frostholm for severe health anxiety 2016 Fogelkvist, Patients with an ED Parling, Kjellin, receiving treatment & Gustafsson at a specialized ED center in Sweden 2016 Hancock et al. Children and adolescents who met DSM-IV criteria for at least one anxiety disorder

2016 Dixon, Wilson, & Habib

2016 Crosby & Twohig

6 –

1/week –

1/week NA

–

–

8 NA

– – –

1/week

NA 1/week

1/week

– 1/week

1/week, then 1-month FU

1/week, then 1/month NA

NA 1/week NA

1/week

18 18 19

12

NA 12

10

2 individual, 12 group – 10

NA

10

NA 8 NA

12

90 –

90 NA

–

45 120 60

60

NA 60

90

– 90

–

NA

180

NA 60 NA

60

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

Adults with schizophrenia and major depression Veterans with diagnosis of chronic, nonterminal pain condition

30

38.5

96

117 25.6 34.4 15.1

8.36

42.73

41.27 42.47 39.83

36.1

8.79

41.97

100

73

10.68 10.54 10.73

9.1

8.3

48.5

13.3

9

327 100

46.5

52

34.5

128 17.8

29

92.2 87.6 96.2

–

Graduate student Primarily licensed chemical dependency counselors

Psychologists

TAU TAU + mindfulness and acceptance group TAU

Psychologists

7/week

8 Variable

8

1/1 to 2 weeks 1/1 to 2 weeks 2/week

8

Psychologist and 10 graduate student – –

ACT

TAU

ACT + TAU

–

1/week, then 1/2 weeks –

NA

NA

NA

Variable

90

50

50

–

150

NA

60 –

60

1/week

– 1/week

ACT Individualized rehabilitation plan Control (no intervention)

Videoteleconference ACT

– 60

– Therapists with at least Master'slevel psychology training Therapists with 8 at least Master'slevel psychology training – – – –

TAU In-person ACT

–

15.4 mean, – 5.9 SD – – 8 1/week

–

ACT

–

47

93.1

Note. AN = anorexia nervosa; ACT = acceptance and commitment therapy; BMI = body mass index; BPD = borderline personality disorder; CBT = cognitive behavioral therapy; CT = cognitive therapy; DCR = Diagnostic Criteria for Research; DSM-IV = Diagnostic and Statistical Manual of Mental Disorders, 4th Edition; DSM-IV-TR = Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision; EDNOS = eating disorder, not otherwise specified; GAD = generalized anxiety disorder; ICD-10 = International Statistical Classification of Diseases and Related Health Problems, 10th revision; MS = multiple sclerosis; MTAU = medication treatment as usual; OCD = obsessive-compulsive disorder; SSRI = selective serotonin reuptake inhibitor; TAU = treatment as usual; TTM = trichotillomania.

2017 Lytsy, Carlsson, Women who were & Anderzén on sick leave or time-restricted disability pension due to mental illness or a pain syndrome 2017 Palmeir, Pinto- Adult women with Gouveia, & BMI ≥ 25 Cunha receiving nutritional treatment for weight loss 2017 Shawyer et al. Adults with schizophrenia or schizoaffective disorder 2017 Shorey et al. Adults in a residential substance use treatment program

2017 Herbert et al.

2017 Gumley et al.

C.W. Ong et al.

Behaviour Research and Therapy 104 (2018) 14–33

Behaviour Research and Therapy 104 (2018) 14–33

C.W. Ong et al.

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