Less than a minute per eye. Once you hold the chart in front of them, patients immediately comprehend the task and spontaneously read the letters. The test continues until two consecutive errors have been made. The CS score is the value of the final correct letter, minus 0.04 for each incorrect letter prior to the two final consecutive errors.
This chart gives some rough guidance in what scores to expect from your patients:
|Mars test score||Degree of loss|
|0 - 0.48||Profound|
|0.52 - 1.00||Severe|
|1.04 - 1.48||Moderate|
|1.52 - 1.76||Typical for age 60 and older|
|1.72 - 1.92||Typical for middle age and young adults|
Keep in mind that because of binocular enhancement effects, expect an average of 0.15 higher scores under binocular testing conditions when the two eyes have similar contrast sensitivity.
It is very important to encourage your patients to guess even after they say they can no longer read letters, so as to minimize the impact of nonvisual factors, such as personality. For example, patients who tend to be cautious will be reluctant to read letters that appear very faint to them. Without encouraging them to guess, such patients' scores might be lower than would be expected purely on visual factors. Note that encouraging patients to guess is good practice for acuity testing as well!
We absolutely do not recommend scoring the test in any way other than that described in the test's User Manual. The scoring procedure is designed for simplicity and accuracy. The scoring algorithm gives credit "letter-by-letter", and is extremely simple to use. We also do not recommend accepting "C" for "O" (or vice versa or any other ad hoc modifications), because such procedural changes alter the guessing probability from 1/10 in a fashion that can also complicate score interpretation. Note that because Cs and Os are relatively infrequent, scoring will be affected only when the contrast threshold is close to one of these particular letters. Accepting C for O and vice versa as correct responses, makes Os and Cs on the chart easier than the other Sloan letters because for these letters alone, you now have two chances in ten to get it correct on the basis of guessing. It's well known that there are small differences in identifiability even between Sloan letters (which were originally designed for equal legibility), but that is probably true for any optotype symbol set. Another important reason to stick with the User Manual scoring procedure is to insure that test scores are comparable from clinic to clinic, research lab to research lab.
The Pelli-Robson uses triplets of letters at each contrast, and these decline in 0.15 log unit steps. The Mars test declines 0.04 log units in contrast from letter to letter. The scoring of the tests is different, as well. These differences result in the Mars test's increase in accuracy of 28%. There are major physical differences as well. The Pelli-Robson is a large wall chart intended for viewing at 100 cm. The Mars test is handheld, and intended for 40-50 cm viewing distance. You can read more about technical differences between the tests here.
Both tests have 8 lines of 6 letters declining in contrast across and down the chart. Both cover roughly the same contrast sensitivity range. Both use the common Sloan letter optotypes used in the ETDRS acuity charts and many other tests, and both measure peak contrast sensitivity. Because the letter stimuli are virtually identical in the two tests, and because of design features of the scoring system, the results you get with the Mars test will correspond closely (within about 0.01 log unit) to Pelli-Robson scores. However, several different scoring variations are in use with the Pelli-Robson, and which one was used is not always known. Because of that, and improvements in the scoring procedure developed for the Mars CS tests that result in 28% better accuracy, we believe that the Mars test scores provide a better standard.
How large are the letters and numerals on the Mars tests?
Mars letters and numerals are large, subtending 2 degrees at a 50 cm test distance and 2.5 degrees at 40 cm. These are equivalent to 20/480 (6/144) and 20/600 (6/180), respectively. The large size ensures that even patients with low vision can see them. If necessary, for patients with lower visual acuity than this, the test can be administered at closer distances as well.
How does Mars letter contrast sensitivity relate to grating contrast sensitivity that probes different size-tuned channels of the visual system?
Research has consistently shown that large letters are identified by the most sensitive of the visual system's size-tuned spatial frequency channels. The Mars tests accordingly yield the sensitivity of the most sensitive channel, and therefore the peak sensitivity of the visual system as a whole. Mars values correspond most closely to grating contrast senstivity at low spatial frequencies, about 0.5-2.0 cycles/degree.