Closely tied to the placebo, or subject expectancy, effect is observer expectancy effect. This is when an experimenter gets the results (s)he wants through unconscious manipulation. From wikipedia:
"An example of the observer-expectancy effect is demonstrated in music backmasking, in which hidden verbal messages are said to be audible when a recording is played backwards. Some people expect to hear hidden messages when reversing songs, and therefore hear the messages, but to others it sounds like nothing more than random sounds."In terms of medical effects, this unconscious bias can manifest in the assessment of symptoms (usually qualitative). A rash looks 'better', hair looks shinier, a person looks less tired. Regarding testimonial evidence, it's hard to separate observer expectancy from placebo effect since the person is both the subject and the observer but both can result in positive testimony even in the absence of a working product. Evidence-based medicines rely on double-blind studies (ie. the researcher doesn't know who has been treated and who has been given placebo) to separate real effects from observer bias.