**1. Introduction**

Over the last decades, different etiological models of post-traumatic stress disorder (PTSD) have led to the design of a number of psychotherapeutic treatments that all target at reducing PTSD symptoms (e.g. exposure therapy [1], cognitive processing therapy [2], for an overview

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

see [3]). Until recently, clinical guidelines and systematic reviews concluded that patients with PTSD require psychotherapeutic treatments that specifically target the trauma experience [4]. However, recent meta-analyses showed that focusing on the trauma experience may not be generally necessary for successful PTSD treatment [5–7].

Randomized controlled trials (RCTs) have typically been conducted in order to identify those components of complex treatment packages that critically impact symptom improvement (i.e. in placebo-controlled studies and comparative or dismantling studies, see [8]). Recently, however, the general validity of RCTs has been criticized in medical as well as in psychotherapy research, by showing that extra-therapeutic factors (such as blinding of outcome assessors or the sample size) may considerably affect the outcome in RCTs [9–12]. Accordingly, metaanalyses, which attempted to explain variation between effect estimates from individual studies—the so-called between-study heterogeneity—identified a number of moderators of treatment effects in PTSD RCTs [7, 13–16].

This paper summarizes meta-analytic findings, which show that in PTSD outcome research extra-therapeutic factors affect the outcome. These findings relate to two questions in the current debate in PTSD outcome research: first, 'Is there evidence that some PTSD treatments consistently outperform others?' and second, 'Is a trauma focus generally necessary for successful PTSD treatment?'. We will briefly describe the research designs that have been used in order to address the abovementioned questions. Then, we will describe common flaws in meta-analyses and we will use examples from PTSD research in order to show how flaws in meta-analyses may lead to invalid conclusions. And finally, we will summarize how the consideration of relevant moderators may alter the conclusions that may be drawn from metaanalyses of RCTs with respect to the two highlighted research questions.
