Control condition design and implementation features in controlled trials: a meta-analysis of trials evaluating psychotherapy..., 2014, Mohr et al.

Abstract
Control conditions are the primary methodology used to reduce threats to internal validity in randomized controlled trials (RCTs). This meta-analysis examined the effects of control arm design and implementation on outcomes in RCTs examining psychological treatments for depression. A search of MEDLINE, PsycINFO, and EMBASE identified all RCTs evaluating psychological treatments for depression published through June 2009. Data were analyzed using mixed-effects models. One hundred twenty-five trials were identified yielding 188 comparisons. Outcomes varied significantly depending control condition design (p < 0.0001). Significantly smaller effect sizes were seen when control arms used manualization (p = 0.006), therapist training (p = 0.002), therapist supervision (p = 0.009), and treatment fidelity monitoring (p = 0.003). There were no significant effects for differences in therapist experience, level of expertise in the treatment delivered, or nesting vs. crossing therapists in treatment arms. These findings demonstrate the substantial effect that decisions regarding control arm definition and implementation can have on RCT outcomes.

Full text at: https://academic.oup.com/tbm/article-abstract/4/4/407/4562977?redirectedFrom=fulltext

 

This study looked at how the control condition influenced effect sizes in trials on psychotherapy for depression. They found that waiting list control (WLC) and treatment as usual (TAU) were associated with much larger effect sizes. The authors report:

"WLCs and TAU controls produced the largest trial effect sizes in the 0.93– 0.95 range; no treatment, specific factor and placebo controls produced similar results in the moderate 0.35–0.56 range; active comparators, pill placebo, and minimal treatment controls produced small, non-significant results. This range of effect sizes is far greater than the range of effect sizes seen across psychotherapies for depression, underscoring the importance of decisions regarding control conditions on the data resulting from RCTs."​

One limitation of their study is that they seemed to combine studies that tested different treatments (if one treatment is less effective and was mostly compared to WLC or TAU that might have affected the results).