Sentence:  One of the greatest sources of unreliability is the constant error introduced by the observer because of distortion of his perceptions created by his own needs or values. An observer who sharply disapproves of certain leadership practices, for example, will have difficulty in preventing a bias. He will tend to <WSD>overcode</WSD> those categories which best describe his feelings. 
Meaning of the word marked with <WSD> tags:  To use a particular code or set of codes too much.