 The sample sizes for this three-arm study were estimated using non-inferiority design. It was assumed that true detection rate in the reference arm refractionist will be 80 and that in the experimental arms would be identical. The investigators estimated that enrolment of 288 participants in each of the three arms with 2.5 one-sided significance level would yield 85 power to demonstrate a similarity margin of -10, if detection rates are identical. Assuming a design effect of 1.5 for the cluster design, the investigators aimed to enrol 435 participants in each of the arms. For comparing accuracy rates between arms, required sample size was 470 per arm to detect a 10 difference with 80 power, 5 significance level and 1.5 design effect. the investigators recruited a minimum of 500 per arm. 