Because of the complex interconnected nature of the human psyche, psychological phenomena are often comorbid and, much like physiological reactions, tend to occur simultaneously. Although the common link between any two such processes may not be fully understood, their systematic relation can be observed and quantified. An estimate of such a systematic linear relation is the correlation coefficient. This estimate quantifies the linear relation between two observed measurements and provides a standardized form of the direction and magnitude of the relation.

A positive correlation is defined as a direction-consistent relation between variables of interest. For example, the severity of the eating disorder anorexia could be thought of as being correlated with measures of self-imposed restriction. The two concepts may be deemed as positively correlated when the ...