I'm pretty sure we all experienced a situation when you ran an experiment and the results ran against your expectations. What did you do? Did you accept the results and changed your beliefs or double and triple checked the experiment in order to find any flaws that would protect your beliefs from being shattered? How about the opposite situation when an experiment confirmed your expectations? Did you go and double check experimental setup in such case? Tendency of people to favor information that confirms their beliefs is called confirmation bias and may have a very strong influence on product development. More so, because great product developers are passionate and opinionated people on a quest to change the world. They live in a world of uncertainty and any confirmation that they are on the right path is highly appreciated. But self-delusion is perhaps the greatest hazard of product development and product managers should work hard to avoid the trap of pushing their agenda instead of acting as representatives of their (potential) users. The best way to avoid such trap is to adopt objective and rigorous experimental methodology that captures true sentiment of users.