1.6 Observer effect and blinding researchers
What if the researchers assessing the outcomes knew the diet allocated to each patient? Perhaps surprisingly, this can have an (unconscious) impact on the values of the response variable. This is called the observer effect (or, in experiments, sometimes called the experimenter effect). This could also compromise the internal validity of the study.
The impact of the observer effect can be minimized by blinding the researchers so that they do not know: which treatments the individuals are receiving. That is, the people giving the treatment and the people evaluating the treatment do not know what treatment has been given. Instead, a third party can be used.
For example, the researchers may give an assistant two drugs labelled A and B. The assistant then administers the drug and evaluates the participants' response to the treatments. Later, the assistant tells the researchers whether Drug A or Drug B performed better, but only the researchers know what drugs the labels A and B refer to.
The observer effect does not just apply to situations where people are used as participants.
Example 1.14 (Observer effect) 'Clever Hans' (https://en.wikipedia.org/wiki/Clever_Hans) was a horse that seemed to be able to perform simple mental arithmetic. After much study, Carl Stumpf realised that the horse was responding to involuntary (and unconscious) cues from the trainer. This was discovered, in part, by using an experiment where the people interacting with the horse were blinded.
The same effect has been observed in narcotic sniffer dogs (Bambauer 2012), who may respond to their handlers' unconscious cues.