The test was BS, and is not highly regarded except among the simple minded; the person administering the "experiment" himself said he influenced the direction of the "experiment" since he played the role of an employee of the prison (superintendent). Impartiality is vital in trying to prove something scientifically.
The BBC Prison study in 2002 is far and wide more reliable and accurate. Zimbarado's attempt is laughable and had too many uncontrollable elements to provide anything remotely "scientific".
Regardless of the fact that Dr. Zimbardo failed to use control groups, the experiment proved to be a detriment to all involved in the experiment within only 6 days out of a proposed 2 week length.
Those who were granted authority were embelleshing upon the rules in which to control the prison populace, in any way in order to maintain authority. Meanwhile, the prisoners became more subserviant, selfless and mindless.