A new look at the construct validity of the K6 using Rasch analysis.

The Kessler six-item psychological distress (K6) scale is widely used to screen for mental disorders; however, information is lacking on the rating scale performance or dimensionality structure of the scale. This study used a population based sample (n = 7596) to evaluate the construct validity of the K6 scale using Rasch partial credit analysis. The analysis showed that almost all of the five-point rating scales in the K6 items were used appropriately to differentiate psychological distress of the study participants. The analysis provided evidence of unidimensionality of the scale, although items 1 (so sad) and 3 (restless or fidgety) might offer a potential second off-dimensional component. All items appeared to fit the Rasch model's expectation as demonstrated by the acceptable item fit statistics. The study participants demonstrated valid response patterns when answering K6 items, except for some who were younger or had higher psychological distress. This study using Rasch analysis confirms the construct validity of the K6 scale and suggests that the K6 is a useful and valid instrument for assessing psychological distress in the mid-aged general population. Further research can facilitate better understanding about the unidimensionality of the scale.