Figure 2. Unidimensionality.

Unidimensionality: This figure shows a hypothetical six-item rating scale, which is unidimensional because the symptoms represented by the items appear in an orderly fashion as the severity of the syndrome increases, such that scoring on higher prevalence items (less severe items) precedes scoring on lower prevalence items (more severe items). When a rating scale is unidimensional, each individual item adds unique information about the severity of the latent syndrome being rated (represented by the dashed frame) and the total score obtained by adding the individual item score is therefore a valid measure for the severity of the latent syndrome (for instance psychotic depression), provided that the scale is also clinically valid (see figure 1).