Besides measures of students' performances, a valid assessment of students' efficiency in a teaching process should also include measures of invested mental effort. The research presented herein covers several approaches in measuring students' mental effort including 5, 7 and 9-point Likert type scales, time on task, as well as eye tracking technique which in combination with performance measures provide valid information on students' success. Results of the research showed good correlation between mental effort assessed by 7 and 5-point Likert type scales and students' performance, while the use of the 9-point scale showed a low degree of correlation, thus recommending the use of a scales with 5 and 7 points for educational purposes over 9-point scales. The research presented herein illustrates how eye tracking can be used to support the evaluation of invested mental effort. Additionally, this method enabled the identification of some student difficulties in the analyzed area -- Stereochemistry. [For the full proceedings, see ED619611.]