An intentional bias occurs when a person or group deliberately alters data in order to change the results of an experiment or study. This type of bias influences the information gathered to go in a certain and predetermined direction. To be an intentional bias the alteration of data and results must be deliberate.
An example would be using desirable results from participants many times during one study in order to make the results seem more robust. This is considered a very immoral thing to do in the scientific community and in research.