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International Journal of Epidemiology, Vol 26, 1041-1048, Copyright © 1997 by International Epidemiological Association
J Katz, SS Yoon, K Brendel and KP West Jr
BACKGROUND: The purpose of this study was to estimate the bias and design
effects associated with the Expanded Program on Immunization's (EPI)
sampling design when estimating xerophthalmia prevalence, and to estimate
the savings associated with EPI in terms of distance travelled within
selected clusters. METHODS: Computer simulation of the EPI sampling
strategy was done using maps from a xerophthalmia survey of 40 wards in
Sarlahi district, Nepal. Samples of fixed cluster sizes of 7, 10, 15, 20
and 25 were compared. The estimated prevalence using the EPI design was
compared with the true prevalence in the 40 wards to estimate the bias. The
design effect was estimated by taking the ratio of the 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 skipping 1-4 houses between
selected ones. The difference between the distance travelled within
clusters using SRS compared with EPI was weighed against the bias and
increased variance. RESULTS: The prevalence of xerophthalmia was 2.8%. The
EPI design overestimated xerophthalmia prevalence by between 0.27% and
1.16%. The design effects of EPI cluster sampling within wards varied
between 0.73 and 1.35. Neither the bias nor the design effect was related
to distance between households or cluster size. Distance travelled within
wards was always less for EPI designs with cluster sizes of 7 or 10. There
was no saving in terms of distance travelled for designs with cluster sizes
from 15 to 25 if there were two or more houses between selected ones. For
fixed cluster sizes of 15 or fewer, the EPI sampling design using nearest
or next nearest neighbours is a better choice than SRS in terms of
minimizing the distance 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
per cluster would be favoured over examining all children in selected wards
unless the travel time between wards was more than 2 days.
ARTICLES
Sampling designs for xerophthalmia prevalence surveys
Department of International Health, Johns Hopkins School of Hygiene and Public Health, Baltimore, MD 21205-2103, USA.
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