Nomothetic Versus Idiographic

The nomothetic (from the Greek term for lawgiving) approach to science seeks lawfulness by testing hypotheses. It applies research-supported general formulations to particular cases and uses deductive reasoning. The idiographic (from the Greek term for oneself, one's own) approach seeks lawfulness by inspecting individual cases and accounting for them; it uses inductive reasoning. General rules are inferred from individual cases.

The nomothetic approach to assessment interprets measurements and observations by comparing them with group norms. The individual case is understood as an instance of a hypothetical general case. The idiographic approach seeks to understand measurements and observations as a function of the individual's history and environment. The individual case is understood in its own context. For example, a newborn might weigh 5 pounds at birth. Nomothetic data show that this is in the 2nd percentile; since 6% of babies weigh too little for their own good, this weight may be cause for concern. Idiographic data—which may include behavioral and other physiological observations of the infant, along with information about the due date—can be used to understand whether this particular baby needs intervention. Nomothetic and idiographic data can thus be used integratively.

Psychological science currently favors the nomothetic approach over the idiographic. A study that shows that, in general, siblings placed together in foster care fare better than siblings placed apart is seen these days as more scientific than a study that examines whether a particular sibling pair should be placed together or apart. Psychological assessment is similarly inclined at this time, with behavior analysis and projective testing disfavored in comparison with scores on scales that have meaning only in relation to normative data. The nomothetic approach decreases variation associated with any given psychologist, but it applies to the individual case only roughly. The idiographic approach, conversely, highlights the case at hand but depends for validity on the particular psychologist.

No assessment device or strategy is entirely nomothetic or idiographic. Even personality inventories, whose individual items are ignored by psychologists in favor of scaled scores, show idiographic features when the psychologist considers whether a particular individual is adequately represented by group norms. Sources of inadequate representation might include some distinguishing feature of the individual or of the situation or of the way the test was administered. Can a personality test normed on the general population be used to categorize or describe a woman of Icelandic heritage? A man recently diagnosed with diabetes? Custody litigants?

Even purely idiographic, unnormed assessment techniques, such as asking about childhood memories or about hobbies, reveal nomothetic features when psychologists consider whether their own personal or cultural backgrounds are affecting their interpretations. In this context, cultural competence might be defined as awareness of one's personal norms and their potential inapplicability to other people.