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Frequency of positive responses on questionnaire as a function of 30-Hz cone electroretinogram (ERG) amplitude for 586 patients with retinitis pigmentosa. Groups were defined based on quintile of ERG amplitude: <0.34 μV (n=132); 0.34 to 0.67 μV (n=125); 0.67 to 1.40 μV (n=117); 1.40 to 4.53 μV (n=109); and >4.53 μV (n=103). (From Berson, et al., 1993).
