In psychology, projective tests presuppose that people have unconscious and implicit motivations and thoughts. Hence, the goal of projective tests is to get at this unconscious level of a person. In order to tap into this area, projective tests give ambiguous stimuli to see how people respond. The most famous of these projective tests is the Rorschach inkblot test.
The reason for the ambiguous stimuli is because patients do not know how to respond to things that are ambiguous. In other words, ambiguous stimuli get behind the socially accepted or learned ways to respond to known stimuli.
In theory, projective tests sound great, but there are a few criticisms. The chief criticism is that there is little control. All the data has to be interpreted and who is to say that this psychologist will interpret the data well? With the lack of objective controls, their charge is always open to radical subjectivity.