The Perceived Stress Scale (PSS, Cohen & Williamson, 1988) is a widely used measure of self-report of perceived stress. Principal components factor analysis often results in a two-factor solution composed of either positively worded (PW) or negatively worded (NW) items (see Lee [2012]). Cohen and Williamson stated that the distinction between factors was irrelevant for purposes of measuring perceived stress.

For this reason, data tends to be viewed from a unidimensional standpoint. The purpose of this study was to analyze responses to the PSS using unidimensional and multidimensional graded response model IRT and to compare the estimated item parameters and trait scores. Responses to the 10-item PSS were gathered from 1,388 participants using a voluntary sample.

The multidimensional model had a significantly better fit to the data over the unidimensional model.  On average, item parameters were estimated with smaller standard errors from the unidimensional model. Scores from the unidimensional and multidimensional model were not significantly different for the entire sample, but were within subgroups.

Males had a significantly different estimated score on the two dimensions from the multidimensional model; the score on the negative dimension was significantly different from the unidimensional score, but the score on the positive dimension was not. Females did not have significantly different scores on the two dimensions from the multidimensional, but each was significantly different from the unidimensional score.

Within different age categories, there was no significant difference in the two trait scores from the multidimensional model, but there were significant differences in unidimensional scores those on one of the two dimensions for certain groups.

Though the unidimensional model is most often applied to psychometric scales with PW and NW items, a multidimensional model may provide a more valid measure of a construct when responses are influenced by the wording direction.