What is the difference between a scientific and an unscientific experiment? My own view is that the Stanford experiment was an unscientific experiment for the following reasons;
(1) The principal investigator, Philip Zimbardo, participated in the experiment as the prison warden. In a good controlled experiment the investigator should not participate in the experiment, there is too much potential for bias.
(2) The initial objective of the experiment will to look at how prisoners become conditioned to the prison system, and the guards were not being monitored as closely as the prisoners, so any conclusions about abuse of power are dubious.
(3) Volunteers were paid for their participation, and some of them really needed the money, this can be a significant source of bias.
(4) There may have been a selection bias problem since the initial advertisement mentioned the experiment was about "prison life", subsequent studies have shown that this phrase may have attracted volunteers with a tendency towards abusive behaviour.
(5) In Zimbardo's book "The Lucifer Effect" he mentions that one of the instructions he gave to the guards at the start was to be like the guards in "Cool Hand Luke" who were sadistic and brutal, this suggests that the guards behaviour was as a result of the Milgram Experiment effect.
(6) A participant asked to leave the experiment; he should have been let go immediately, instead Zimbardo got him to stay, and this action led the prisoners to think that they couldn't exit the experiment -- a fundamental principle of informed consent is that participants cannot be forced continue in an experiment.