Measurement invariance in the enjoyment of reading across grades measured by IRT and DIF analysis

Abstract When measuring a phenomena across different grades it is important that the instrument measures the same way by being perceived in the same manner no matter what grade. Especially if the phenomena is a concept that should be universal and therefore independent of other circumstances, such as age. To the best of all worlds, the phenomena of enjoyment of reading should thus be universal and measure the same across grades through its measurement of intrinsic motivation of reading. Inspired by the ILSA study PIRLS 2021 a scale of enjoyment of reading was created and examined for DIF across ages and evaluated by IRT in a sample of all pupils in the public-school system in one municipality in Denmark (grade 0.–9., age 6–16). The analysis shows DIF in items, and the DIF cluster to be similar for younger pupils (0.–2. grade, age 6–8) and the older pupils (3.–9. grade, age 9–16), which indicates that the measurement is different for the two groups. The measurement of enjoyment of reading is thus not universal, as our results indicate that grades 0–2 should not be included or that one should be aware of the potential issues related to these first grades.