Examples Of The Evidence Base

The subsequent chapters in this book will provide numerous specific examples of studies that provide evidence one way or another for the use of particular therapies with particular disorders, but it is worth considering one or two such studies briefly, then considering one or two of the meta-analyses and mega-analyses in order to illustrate some of the more general points that we wish to make about the evidence base.

One of the most famous and most expensive therapy outcome studies was the National Institute of Mental Health (NIMH) Treatment of Depression Collaborative Research Program, which will be considered in order to illustrate the problems that have arisen from the general failure to find differential effectiveness of therapy outcome (see Elkin et al., 1989; Elkin, 1994) but also to illustrate other issues about the evidence base. This trial was the largest of its kind ever carried out. There were 28 therapists working at three sites; eight therapists were cognitive-behavioural, 10 were interpersonal therapists, and a further 10 psychiatrists managed two pharmacotherapy conditions, one being imipramine plus 'clinical management', the second being placebo plus 'clinical management'. Two-hundred-and-fifty patients meeting the criteria for major depressive disorder were randomly allocated between the four conditions. The therapies were manualised and considerable training and supervision occurred both before and throughout the trial by leading authorities for each therapy (see Shaw & Wilson-Smith, 1988, for a detailed account of this process). Elkin et al. (1989) reported that all four groups improved approximately equally well on the main symptom outcome measures. Perhaps the most surprising result was the extent of the improvement in the placebo-plus-clinical management group, which substantially outperformed control groups in most other studies, although a post hoc analysis showed that it was less effective for patients with more severe depressive disorders. Imber et al. (1990) have further shown that there were no specific effects of treatments on measures such as the Dysfunctional Attitude Scale on which, for example, the cognitive therapy condition would have been expected to make more impact than the other treatments. In summary, the NIMH trial illustrates that although it has been important to test treatment effectiveness, the simple comparison of outcome of treatment is the most expensive and least informative way in which to approach the issue.

To move now to an example of meta-analysis, we will start with the Robinson et al. (1990) study, which has been widely cited and is the most influential meta-analysis in the area of depression. Robinson et al. identified 58 trials of a comparison of psychological therapies and a further 15 trials of psychological therapy versus pharmacotherapy that were published between 1976 and 1986. The statistical combination of these studies gave an effect size of 0.73 for psychotherapy versus control (as a reminder, an effect size significantly greater than zero shows a positive benefit, with an effect size of >0.7 being considered to be a large effect). Other comparisons revealed a benefit of cognitive therapy over non-CBT therapies of 0.47, and of other cognitive-behavioural therapies over non-CBT therapies of 0.27. This meta-analysis has often been quoted as showing a distinct small-to-moderate benefit of cognitive and cognitive behavioural approaches over psychodynamic approaches in the treatment of depression. However, there are a considerable number of qualifications to this apparently straightforward conclusion. First, only nine of the 58 trials were based on standard clinical recruitment, with 50 % of trials recruiting participants through the media, 25 % of trials being student based, and with only 35 % having inclusion criteria for clinical depression. In addition, most of the studies reported only post-treatment data without follow-up data being included. Although more recent studies and more recent meta-analyses (such as Gloaguen et al., 1998) generally have stricter inclusion criteria and include follow-up data, it is important to note the limitations of many of the earlier studies and the earlier meta-analyses.

A second type of approach for combining quantitative data has begun to appear - this is the so-called mega-analysis in which case-level data from several studies are combined in order to provide statistical power for more sophisticated analyses. For example, Thase et al. (1997) combined data from six different studies to give a total of 795 participants who had received cognitive behaviour therapy or interpersonal psychotherapy alone or combined with an anti-depressant. Their analyses showed that there is a benefit for combined drug-psychotherapy treatment for more severe levels of depression, but for mild to moderate levels of depression there was no advantage for combined treatment.

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