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Pelli Robson Test
Availability
Charts are available for purchase: Click here for Pelli-Robson Test.
Classification
Supplemental: Multiple Sclerosis (MS) and Parkinson's Disease (PD)
Short Description of Instrument
Construct measured: Contrast sensitivity.
Generic vs. disease specific: Generic.
Means of administration: Observation.
Intended respondent: Patient.
# of items: N/A.
# of subscales and names of sub-scales: N/A.
# of items per sub-scale: N/A.
Comments/Special Instructions
Scoring: The subject is assigned a score based on the contrast of the last group in which two or three letters were correctly read. The score, a single number, is a measure of the subject's log contrast sensitivity. Thus a score of 2 means that the subject was able to read at least two of the three letters with a contrast of 1% (contrast sensitivity = 100 % or log 2). A Pelli-Robson score of 2.0 indicates normal contrast sensitivity of 100 %. Scores less than 2.0 signify poorer contrast sensitivity. Pelli-Robson contrast sensitivity score of less than 1.5 is consistent with visual impairment and a score of less than 1.0 represents in visual disability. This score (1.0) represents an approximately 10-fold loss of contrast sensitivity. That is, a person with contrast sensitivity of 1.0 requires 10 times as much contrast to see as compared with a person with normal vision.
 
Background: The Pelli-Robson test measures contrast sensitivity using a single large letter size (20/60 optotype), with contrast varying across groups of letters. Specifically, the chart uses letters (6 per line), arranged in groups whose contrast varies from high to low.
References
Pelli DG, Robson JG, & Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clinical Vision Sciences. 1988; 87-199
Rationale/Justification
Strengths/Weaknesses: May be useful for predicting the threshold visibility, such as of large trucks in the fog but not for determining the presence of small objects such as the child on the street.
 
Psychometric Properties: Inter-rater agreement was described with the intraclass correlation coefficient (ICC) and comparison of mean scores. Excellent inter-rater agreement (ICC=0.86-0.95) was demonstrated at each contrast level among MS patients (n=100) and visually-asymptomatic volunteers (n=33). Average letter scores at the lowest contrast level (0.6%) were highly variable in the MS group, even among patients with visual acuities of 20/20 or better, and among those who required no assistance for ambulation.
 
Administration: Patients read the letters, starting with the highest contrast, until they are unable to read two or three letters in a single group. Each group has three letters of the same contrast level, so there are three trials per contrast level.