One of the core features of science (and therefore science-based medicine) is to precisely identify and control for variables, so that we know what, exactly, is exerting an effect. The classic example of this principle at work is the Hawthorne effect. The term refers to a series of studies performed between 1924 and 1932 at the Hawthorne Works. The studies examined whether or not workers would be more productive in different lighting conditions. So they increased the light levels, observed the workers, and found that their productivity increased. Then they lowered the light levels, observed the workers, and found that their productivity increased. No matter what they did, the workers improved their productivity relative to baseline. Eventually it was figured out that observing the workers caused them to work harder, no matter what was done to the lighting.
This “observer effect” – an artifact of the process of observation – is now part of standard study design (at least well-designed studies). In medical studies it is one of the many placebo effects that need to be controlled for, in order to properly isolate the variable of interest.
There are many non-specific effects – effects that result from the act of treating or evaluating patients rather than a physiological response to a specific treatment. In addition to observer effects, for example, there is also the “chearleader” effect from encouraging patients to perform better. There are training effects from retesting. And there are long-recognized non-specific therapeutic effects just from getting compassionate attention from a practitioner. It is a standard part of medical scientific reasoning that before we ascribe a specific effect to a particular intervention, that all non-specific effects are controlled for and eliminated.