Concepts and their measurement
Psychological concepts are typically fuzzy because the boundaries of psychological concepts are not sharp. It could even be argued that psychological concepts such as intelligence, personality, and self-constructs such as self-esteem do not exist per se, but rather that they are constructed. Measurement of psychological constructs, therefore, presents a particular challenge.
In order to measure a fuzzy concept, it is generally a good idea to define the construct as clearly as possible. Then the construct can be more readily operationalised. Once the construct has been operationalised, the measure should be trialled, with analysis of gathered data to assess the measure's reliability and validity. This is the process of psychometric instrument development.