Studies have demonstrated that task-evoked pupillary responses (TEPRs) can be adopted to measure the examinee's cognitive load. This study compared three approaches for the measurement of TEPRs, mean pupil diameter, mean pupil dilation, and mean percentage of pupil dilation, to determine the best-fit measuring method. The valid participants of this study were eight sixth-grade elementary students. The experimental materials used were two mathematics questions with differing difficulty. The generalized estimating equation (GEE) was employed to compare the goodness of fit of each approach. The results revealed that the measurement of TEPRs based on the mean percentage of pupil dilation measured every 4 s provided the best fit.