SAMPLING DESIGNS FOR XEROPHTHALMIA PREVALENCE SURVEYS

Background. The purpose of this study was to estimate the bias and des ign effects associated with the Expanded Program on Immunization's (EP I) sampling design when estimating xerophthalmia prevalence, and to es timate the savings associated with EPI in terms of distance travelled within selected clusters. Methods. Computer simulation of the EPI samp ling strategy was done using maps from a xerophthalmia survey of 40 wa rds in Sarlahi district, Nepal. Samples of fixed cluster sizes of 7, 1 0, 15, 20 and 25 were compared. The estimated prevalence using the EPI design was compared with the true prevalence in the 40 wards to estim ate the bias. The design effect was estimated by taking the ratio of t he variance under EPI sampling to that of stratified random sampling ( SRS) with fixed cluster sizes. The EPI was also modified by increasing the distance between selected houses from nearest neighbour to skippi ng 1-4 houses between selected ones. The difference between the distan ce travelled within clusters using SRS compared with EPI was weighed a gainst the bias and increased variance. Results. The prevalence of xer ophthalmia was 2.8%. The EPI design overestimated xerophthalmia preval ence by between 0.27% and 1.16%. The design effects of EPI cluster sam pling within wards varied between 0.73 and 1.35. Neither the bias nor the design effect was related to distance between households or cluste r size. Distance travelled within wards was always less for EPI design s with cluster sizes of 7 or 10. There was no saving in terms of dista nce travelled for designs with cluster sizes from 15 to 25 if there we re two or more houses between selected ones. For fixed cluster sizes o f 15 or fewer, the EPI sampling design using nearest or next nearest n eighbours is a better choice than SRS in terms of minimizing the dista nce travelled and the mean square error. Conclusions. The choice of an optimum method would need to account for the density of clusters and difficulty of travel between clusters, as well as the costs of travel within clusters. Based on certain assumptions, EPI with 15 children pe r cluster would be favoured over examining all children in selected wa rds unless the travel time between wards was more than 2 days.