ABSTRACT Purpose To develop a model investigating the effects of the psychometric function's slope and lapse rate on QUEST visual acuity (VA) thresholds for preschool children. Methods Monte Carlo simulations of QUEST (16 trials) measurements of VA were performed using an underlying ideal observer model for a range of Ideal Thresholds, Slopes, and Lapse Rates for 10‐ and 4‐alternative forced choices (AFC). Ideal Slopes (in probit size) were varied from steep to flat, 0.05 to 0.20 logMAR. Ideal Lapse Rates were varied from 0.01 to 0.50 (at Ideal Probit Size of 0.10 logMAR). Thresholds were estimated using three different techniques: (i) QUEST's maximum likelihood estimate (75% correct), and probit analysis of logMAR value (55% correct) with (ii) fixed‐slope probit (probit size = 0.10 logMAR) or (iii) floating‐slope probit (probit size = 0.02–0.29 logMAR). An index, Gain‐Corrected Standard Deviation, was developed to assess how sensitive a technique was for detecting a change in VA. A smaller Gain‐Corrected Standard Deviation indicates a technique is more sensitive to change. Results Measured VA thresholds from QUEST, floating‐slope probit, and fixed‐slope probit are linearly related to Ideal Threshold from −0.20 to 0.80 logMAR. Flat slopes and higher lapse rates result in an increased Gain‐Corrected Standard Deviation for all threshold measurement techniques (higher when thresholds were estimated with probit fitting than with QUEST Maximum likelihood), with the increase in Gain‐Corrected Standard Deviation being slightly higher for 4AFC compared to 10AFC across the range of Ideal Slopes and Lapse Rates. For 4AFC, the Gain‐Corrected Standard Deviation range for varied lapse rates (0.01–0.50) was QUEST, 0.062–0.567; floating‐slope probit, 0.079–0.623; and fixed‐slope probit, 0.079–0.626 logMAR. Conclusions QUEST maximum likelihood outputs should be used instead of fitting probit functions in preschool children when there are limited numbers of trials. Flat slopes and high lapse rates can significantly decrease the precision of QUEST estimates of VA in preschool children.
Suwal et al. (Thu,) studied this question.