Klaas Sijtsma is a professor of Methods of Psychological Research at the Tilburg School of Social and Behavioral Sciences, Tilburg University, the Netherlands. His research specializes in psychometrics, in particular, all issues related to the measurement of psychological attributes by means of tests and questionnaires. He is a past President of the Psychometric Society, associate editor of Psychometrika, editorial board member for other journals, and has authored several books on measurement.

Andries van der Ark is professor of Psychometrics at the Research Institute of Child Development and Education. His primary research interests include reliability analysis and nonparametric item response theory. The authors have published over 40 papers together on measurement in the social and behavioral sciences.