Observer bias

Subjective assessments are quick and easy to obtain. They give answers to questions which are difficult to answer otherwise. Unfortunately subjective assessments suffer from two kinds of observer bias. First, the assessment selected tends to be closer than it should be to the middle of the range of available assessments. This is called a range effect. Secondly, a general subjective assessment not tied to a specific task may be determined by common knowledge instead of by the details of the question under consideration. If so, the assessment may give a wrong answer. Clearly, subjective assessments should be used only when it is impossible to obtain valid objective measures of performance. They need to be interpreted with extreme caution.