There are several possible explanations for the surplus of eyes flagged as showing progression by NPA. First, as explained in the introduction, GPA cannot be used in advanced stages of glaucoma. Glaucoma progression analysis labels test locations that are already severely affected at baseline with an “X” on the printout. These locations are not further addressed by GPA because it is unable to determine whether any change from baseline is significant (ie, any further deterioration lies within the range expected from long-term fluctuation). Nonparametric progression analysis may perform better because it includes these points out of range in GPA. Likewise, in eyes with a substantial number of severely affected locations, GPA gives a reading of baseline MD out of range, which occurs when the mean MD of the 2 baseline fields is worse than −15 dB. Nonparametric progression analysis can still assess these eyes. Indeed, in eyes flagged by NPA only, a worse MD was found on average, and GPA more often gave a reading of baseline MD out of range, compared with eyes flagged by GPA. After excluding eyes with baseline MD out of range, agreement between NPA and GPA improved. Second, because of its design, GPA is insensitive to a general decrease in sensitivity. Development of cataract presumably is a common cause of a general decrease in sensitivity. Insensitivity to cataract obviously is an advantage in a glaucoma trial. However, in a clinical setting, patients may benefit from the fact that a clinician has to evaluate the lens before he or she can interpret perimetry results. Third, glaucoma causes diffuse loss in addition to localized deterioration.12 Glaucoma progression analysis will not detect such diffuse loss. Some eyes flagged by NPA only might have diffuse loss, but because a standard criterion for progression is lacking, it is impossible to determine how many. Fourth, some eyes flagged by NPA only could represent false-positive classifications. The specificity of NPA has been calculated to be 0.83 for possible progression, 0.90 for likely progression with 2 confirmations, 0.93 with 3 confirmations, and 0.95 with 4 confirmations.15 If we assume a specificity of 0.83, then 27 of 89 eyes flagged by NPA would represent false-positive classifications (estimated from TP + [221 − TP] × [1 − specificity] = 89, where TP is the number of true-positive classifications). Obviously, this number of false-positive classifications is an upper estimate because most eyes flagged by NPA were confirmed more than once. Hence, even without knowing the false-positive classification rate of GPA in this study, it can be concluded that false-positive classification alone cannot explain the number of eyes flagged as showing progression by NPA but not by GPA.