Village and household clustering of xerophthalmia and trachoma.

The clustering of xerophthalmia in villages and in households was assessed among preschool children surveyed in the Lower Shire Valley, Malawi, and in Aceh province, Indonesia. Trachoma clustering was similarly assessed among the same children in Malawi. Trachoma clustered much more than xerophthalmia among villages and among households. The impact of xerophthalmia clustering on sample size considerations for future surveys or interventions was similar in Malawi and Indonesia. Village clustering of xerophthalmia would necessitate a twofold increase in sample size. Household clustering in the absence of village clustering would have almost no impact on sample size. Village clustering of trachoma would necessitate a ninefold increase in sample size. Household clustering would increase sample size requirements by 26%.