The nomothetic vs. the idiographic approaches.
Nomothetic approach- Researchers assume that all people can be described along a single dimension according to their level of, for example, assertiveness or anxiety. They are tested to see how their scores for the trait compares with scores of others. Traits that apply to everyone are called common traits. Indispensable for understanding personality. VS Idiographic approach- rather than forcing people into categories selected beforehand, researchers identify the unique combination of traits that best accounts for the personality of a single individual.