What was the major flaw in the stanford prison experiment?

The major flaw in the Stanford Prison Experiment is that the author of the study, Dr. Zimbardo, participated in the study. He took part of the study by participating as the head guard of the prison. This was considered as a flaw because his action may have led to creating unnecessary pressure to the participants who happened to be his students as well. 

raymond can buy special closed cooling containers or a semi-truck with a air conditioned cooling system and an open box for the fruits,

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