Nonparametric IRT analysis of Quality-of-Life Scales and its application to the World Health Organization Quality-of-Life Scale (WHOQOL-Bref).
BACKGROUND
This study investigates the usefulness of the nonparametric monotone homogeneity model for evaluating and constructing Health-Related Quality-of-Life Scales consisting of polytomous items, and compares it to the often-used parametric graded response model.


METHODS
The nonparametric monotone homogeneity model is a general model of which all known parametric models for polytomous items are special cases. Merits, drawbacks, and possibilities of nonparametric and parametric models and available software are discussed. Particular attention is given to the monotone homogeneity model (also known as the Mokken model), and the often-used parametric graded response model.


RESULTS
Data from the WHOQOL-Bref were analyzed using both the monotone homogeneity model and the graded response model. The monotone homogeneity model analysis yielded unidimensional scales for each content domain. Scalability coefficients further showed that some items have limited scalability with respect to the other items in the same scale. The parametric IRT analyses lead to the rejection of some of the items.


CONCLUSIONS
The nonparametric monotone homogeneity model is highly suited for data analysis in a health-related quality-of-life context, and the parametric graded response model may add interesting features to measurement provided the model fits the data well.
Sijtsma
Klaas
K
Department of Methodology and Statistics FSW, Tilburg University, PO Box 90153, Tilburg 5000 LE, The Netherlands. k.sijtsma@uvt.nl
Emons
Wilco H M
WH
Bouwmeester
Samantha
S
Nyklícek
Ivan
I
Roorda
Leo D
LD
D006305:Health Status Indicators; D006801:Humans; D015233:Models, Statistical; D011788:Quality of Life; D018709:Statistics, Nonparametric; D011795:Surveys and Questionnaires; D014944:World Health Organization