Item analyses of the 20-item prosopagnosia index using classical test theory and item response theory.

The 20-item prosopagnosia index (PI20) is a highly practical tool for assessing lifelong difficulties in face processing across individuals (developmental prosopagnosia). Recent research suggests that the quality of the PI20 may vary depending on the item; however, the quantitative quality of each item has not been sufficiently examined. In this study, the item properties of the PI20 were investigated using classical test theory and item response theory. The analyses confirmed that the one-factor structure of the PI20 showed high reliability and validity. Item response theory further revealed that the items differed in measurement power, discrimination, and difficulty under the unidimensional graded response model. Items reflecting the additional psychological load and social consequences associated with face processing difficulty demonstrated high measurement precision. In contrast, items related to the recognition of distinctive or self-faces contributed to the construct to a lesser degree. These results indicate that while most items adequately capture the characteristics of developmental prosopagnosia, certain items may not be suitable for screening for developmental prosopagnosia. The present study provides open materials to facilitate further psychometric evaluation of the PI20 and offers insights for refining self-report assessment of developmental prosopagnosia.