In the VIP Study, demographic information (birth date, gender, race and ethnicity) of a child was collected at enrollment based on information provided by the child’s parent or legal guardian. For easier comparison with other studies Race/ethnicity was classified as American Indian, Asian, Black, non-Hispanic White, Hispanic, and other/unknown (for those reported with more than one race categories or those without race information). Age was calculated as the difference between date of comprehensive eye examination and birth date, and was grouped as 36–47 months, 48–59 months, and 60–72 months.
Ocular risk factors were defined based on findings from comprehensive eye examinations. Strabismus status was classified as esotropia, exotropia, or no horizontal strabismus. To facilitate comparison of our findings with those from other studies, we defined the presence and severity level of the ocular risk factors similarly to other studies.6,14 For the ocular risk factors of unilateral amblyopia, we determined each type of refractive error based on the worse eye, as we assumed that the ocular condition in the worse eye dominated the association with unilateral amblyopia. We classified presence and severity levels for myopia (<0.5 D, ≥0.5 to <2 D, ≥2 D), hyperopia (<2 D, ≥2 to <3 D, ≥3 to <4 D, ≥4 to <5 D, ≥5 to <6 D, ≥6 D), astigmatism (<1 D, ≥1 to <2 D, ≥2 to <3 D, ≥3 to <4 D, ≥4 D), and spherical equivalent anisometropia (<0.5 D, ≥0.5 to <1 D, ≥1 to <2 D, ≥2 D). For the ocular risk factors of bilateral amblyopia, we defined bilateral astigmatism and bilateral hyperopia based on the better eye. Because bilateral amblyopia is a condition affecting both eyes, we required that both eyes should have the refractive error to qualify it as an ocular risk factor for bilateral amblyopia.