Why do the results of zimbardo's prison study not lead to a firm conclusion?

The results of Zimbardo's prison study do not lead to a firm conclusion for two reasons:
a. First of all, the experiment was intended to go on for two weeks. However, due to the experiment going out of hand, it had to be stopped within 6 days. This was because many of the people who were paying the role of prisoners started getting hit and abused. Some  even developed psychosomatic rashes, others started suffering from other psychological disorders.
b. Many critics believe that the results were not very reliable since Zimbardo himself was part of the experiment (he  played the role of the prison warden) and could have influenced the direction of the experiment.