Validity of the FTGOT scores was established by means of some qualitative and quantitative methods. These are: (1) content and face validations by six experts; (2) Positive correlations between the PSPTs' correct scores considering only first tiers of the FTGOT and their confidence score for this tier (r =  .194) and between correct scores considering first and third tiers and confidence scores for both of those tiers (r =  .218) were found as evidences for construct validity. (3) False positive (3.5%), false negative (3.3%) and lack of knowledge (5.1%) percentages were found to be less than 10% as an evidence for content validity of the test scores. (4) Explanatory factor analysis conducted over correct and misconception scores yielded meaningful factors for the correct scores as an evidence for construct validity. Additionally, the Cronbach alpha coefficients were calculated for correct scores (r = 0.59) and misconception scores (r = 0.42) to establish the reliability of the test scores. Six misconceptions about geometrical optics, which were held by more than 10% of the PSPTs, were identified and considered to be significant.

The results from the present investigation demonstrate that the FTGOT is a valid and reliable instrument in assessing misconceptions in geometrical optics.