There has been a recent surge of interest in evidence-based medicine, which has led one or two sceptics to question what medicine was based on before. The answer is, of course, that medicine has always been based on 'evidence' but maybe not on evidence as we now know it. There was always 'evidence' that the world was flat (you just had to look at the Earth), that the sun rotated around the Earth (you just had to look at the sun moving through the sky) and that blood letting was an effective form of treatment for maladies (some patients did actually recover who might otherwise have died). In a world where we seek to confirm rather than disprove our beliefs there is always evidence to be cited in favour of multiple and contradictory viewpoints. Given the ambiguous nature of evidence, therefore, one of the key questions for a book such as this has to be a consideration of the concept of evidence itself. What counts as evidence? Do different approaches take different views of what counts as evidence? What of apparently contradictory sets of evidence? How can such contradictions be resolved or explained?

There is now a major growth industry in medicine that examines the nature of evidence and draws evidence together in order to produce expert clinical guidelines for the treatment and management of all possible disorders. Most of this work draws on quantitative and qualitative methods for the assessment and grading of evidence in addition to methods for combining evidence across studies. Earlier evidence review methods were of a qualitative nature but there are now quantitative review methods, for example in the forms of meta-analyses and mega-analyses (see later) that allow the combination of evidence from different studies. The increasingly influential Cochrane reviews (named after the epidemiologist, Archie Cochrane, in particular for his influential 1972 book) categorise evidence from different studies into levels: high-quality meta-analyses and randomised controlled trials (RCTs), case-control or cohort studies, case reports or case series, and expert opinion.

Handbook of Evidence-based Psychotherapies: A Guide for research and practice. Edited by C. Freeman & M. Power. Copyright © 2007 John Wiley & Sons, Ltd.

Cochrane reviews and other clinical guideline groups (such as the SIGN groups in Scotland and the NICE groups in England) pool together all published studies that satisfy basic inclusion criteria, but thereby suffer from the publication bias problem that positive results are more likely to be submitted and accepted for publication than are negative results. It is possible to estimate the extent of this bias based on the standard error of published findings (see, for example, Begg & Mazumdar, 1994), although this is rarely done when such reviews are carried out. However, an equally important issue that keeps many psychotherapists awake at night is whether or not the RCT view of evidence is the appropriate one for psychotherapy. Although RCTs provide the gold standard for evidence in many areas of medicine, a number of the requirements for a high-quality RCT are difficult to meet in psychotherapy research. For example, although in theory patients and raters in pharmacotherapy trials may be blind as to whether the person is in the placebo group or the active drug group of the trial, it is nearly impossible to blind patients (and therefore raters) about which arm of a psychotherapy trial they are in. Part of the purpose of this chapter and a number of subsequent chapters will therefore be to consider some of the limitations of the current evidence-based approach to avoid uncritical acceptance of a flawed and complex evidence base while also avoiding the need for its complete rejection.

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