This article reports on a user study conducted to asses the credibility of a humanoid robot. The study set-up was based on the "Monty Hall Problem". Overall 13 people between the ages of 19 and 84 took part in the study (7 male and 6 female). The experiment was set up as a card-game where the participant had to guess which of the three cards shows a price. At one point of the experiment the robot advised the participant to change his/her mind and choose another card. During the user study the participants had to fill in a questionnaire on their level of certainty about their choice and the credibility of the robot. The results showed a significant correlation between the believability of the robot and the certainty in the decision made. Furthermore, the outcomes showed differences between participants who followed the robot's advise and participants who did not, regarding credibility, certainty of the decision made and the estimation whether the robot was helpful or not...