Dear Bradley,
Thank you for the correction. Indeed, this would be an error of precision rather than bias - so depending on how the selected clusters fell, cluster sampling might also over estimate the prevalence.
In the case of the area studied in the article, the selection of clusters happened to fall into the villages with the lower prevalence, leading to the conclusion that the prevalence of the entire zone was so low the NGO pulled their services and redirected them elsewhere. If the cluster selected had been just a half km west, they would have had come to very different conclusions. When the MoH saw the results of this survey, they tried to redirect their own resources to address this cluster, but they lacked the resources to do it properly.
I'm certainly no expert at the nuances of survey design. My understanding is that the current 30x30 cluster sampling is based on accurately detecting prevalences more typical of moderate wasting, that it is not really appropriate for accurate SAM prevalence when the prevalence is fairly low. And if kwashiorkor does cluster more tightly than wasting, as annecdotally appears to be the case, and if prevalence is often as high as the area in this study, then perhaps we need to be working on a different survey design for regions where kwashiorkor is the majority of the SAM cases?
Through out eastern DRC, local health staff can direct you to those villages or groups of villages where kwashiorkor is clustering. In those cases where I have followed up on their directions, they've been correct. In these areas, people are pretty good at detecting not only cases with bipedal pitting edema, but also children that are in earlier stages. Perhaps in cases where the overall prevalence is estimated to be relatively small, say under 4%, would active case-finding provide a more accurate approach? Something like # cases found through active case-finding/total estimated population?
Thanks for the commentary and input,
Merry