The letter and numeral versions of the Mars Contrast Sensitivity Test are identical except for the optotype symbols. Here are some reasons why one might choose to test with number symbols rather than letter optotypes:
• Many cultures use characters other than those in the Roman alphabet used in most Western countries (e.g. those in Asian countries, Arab countries, Greece, and Russia). Numerals (number symbols), on the other hand, are recognized all over the world and used in everyday transactions in virtually all contemporary cultures.
• The distribution of letter frequencies varies substantially depending on language (see https://en.wikipedia.org/wiki/Letter_frequency), leading to potential biases in responding at very low contrasts. Numerals, by contrast, are more “language neutral” in the sense that frequency of occurrence of individual numerals is similar across languages.
• There are only ten Sloan letter optotypes (C, D, H, K, N, O, R, S, V, and Z), but 26 letters in the Roman alphabet. If examinees are presented with Sloan letters but are allowed to give responses that are outside the Sloan letter set, then the assumption of a 1/10 guessing probability is unwarranted, even if those responses are scored as incorrect. Restricting the examinee’s response choices to the ten Sloan letters is one reasonable solution, but this adds time to instructing and testing the examinee. Since there are only ten numerals (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9), instructing the examinee to respond with a numeral is simpler and better supports the assumption of a single consistent guessing probability than does the use of letter optotypes.
Despite these clear advantages, letter optotypes are more common than numeral optotypes in vision testing, and when direct comparison with other tests using letters is desired, the Mars letter version may be preferable.