Response bias occurs when patients in a clinical trial - which may include CFS trials - artificially inflate their scores to please researchers. It is recognised as a potential problem, but there isn't a lot of good research on it. A recent paper from placebo expert Asbjorn Hrobjartsson discusses some of the issues with response bias, as it can artificially inflate the response to placebo treatment too. Placebo effect studies are susceptible to response bias and to other types of biases. Hrbjartsson A, Kaptchuk TJ, Miller FG. (2011) Abstract (highlights) OBJECTIVE: Investigations of the effect of placebo are often challenging to conduct and interpret. The history of placebo shows that assessment of its clinical significance has a real potential to be biased. We analyze and discuss typical types of bias in studies on placebo. ...RESULTS: The inherent nonblinded comparison between placebo and no-treatment is the best research design we have in estimating effects of placebo, both in a clinical and in an experimental setting, but the difference between placebo and no-treatment remains an approximate and fairly crude reflection of the true effect of placebo interventions. A main problem is response bias in trials with outcomes that are based on patients' reports... CONCLUSIONS:Creative experimental efforts are needed to assess rigorously the clinical significance of placebo interventions and investigate the component elements that may contribute to the therapeutic benefit. Background on placebo studies Hrobjartsson famously demonstrated that placebo effects in general have been greatly overestimated. The critical mistake was to assume all improvement from baseline in the placebo group of clinical trials was due to the placebo itself, when there are other explanations including natural improvement and regression to the mean. By comparing placebo groups with more appropriate groups eg waiting list or 'no treatment' groups, he showed that placebos did not generally produce powerful clinical effects. These findings were broadly confirmed in even larger meta-analyses, most recently in 2010. Nonetheless, the placebo effect is not dead, as he points out in this paper: The issue with patient-reported outcomes is where response bias comes in, and is particularly relevant to CFS studies where almost all outcomes are self-reported. The general problem is set up in this paper as: I've picked out key points from the discussion of response bias: Of course, in non-blinded and non-placebo-controlled studies, such as the PACE Trial, this doesn't apply and response bias can indeed distort results. PACE relied on therapy with an average of 13 hours of contact between therapist and patient, and a typical patients-therapist relationship that was independently assessed as 'very strong'. Such a strong patient-provider relationship is likely to be a causal factor in response bias, as this paper spells out.