From SKYbrary Wiki
Confirmation Bias - Once an initial understanding (mental model) has been formed, the individual will search his/her memory and the immediate situation for additional data relevant to that mental model. Such data, then, are apt to be recalled and regarded as pertinent only to the extent that they confirm the mental model at hand. Although potentially confirmatory information tends to be taken at face value, potentially disconfirming information is subjected to a more critical and sceptical scrutiny.
Several studies have shown that preliminary hypotheses formed on the basis of ambiguous data interfere with the later interpretation of better, more abundant data (Lewis and Normal, 1986). An individual’s mental model of a situation is likely to correspond to a large part to the reality, even though it may be wrong in some respects. Having expectations frequently confirmed reduces the sensitivity of the error detection mechanism. Confirmation bias is a selective process that favours information relevant to the presently held view. In essence it is a bias towards relevant-appearing information. Additionally acting upon one’s beliefs can also increase the psychological costs or “dissonance” involved in changing one’s beliefs (Festinger, 1957).
Confirmation bias can have such a strong impact that once individuals have developed a mental model of a problem space, and they have confirmed their model, it becomes very difficult to let go of the model, even in the light of contradictory information. The need for a realignment of one’s mental model becomes apparent only in the light of one or more extraordinary events that do not fit the model.