Epidemiology 1, 2, 3: study and sample design.

Abstract

Abstract. Bad sample designs and selection bias have plagued studies on schistosomiasis, and as a result some believe that schistosomiasis is too focal, making it difficult to draw reliable samples. The Epidemiology 1, 2, 3 (EPI 1, 2, 3) sample design, although complex, demonstrates that sampling theory is readily applicable to epidemiologic studies of schistosomiasis. The EPI 1, 2, 3 sampling scheme was designed to achieve the smallest feasible standard errors given EPI 1, 2, 3 objectives and certain logistical constraints. The sample design is a multi-stage selection of villages (ezbas, which were stratified by size) and households within each of 9 purposely selected Egyptian governorates. Villages and households were systemically selected from census frames. The sampling of ezbas was especially difficult because of the lack of complete sampling frames and their wide variation in population size. Ultimately, ezbas were stratified by size and then randomly selected from each stratum. Sample sizes for villages and ezbas and individuals within ezbas were calculated based on EPI 1 and 2 objectives, respectively. No re-selection was made for non-respondents. A 20% subsample of the full sample was drawn for clinical and ultrasonographic examinations. The sample selected from individual governorates closely parallel the age structure of the 1986 census of the respective rural populations. Details of the study design and related methods are given below.Epidemiology 1, 2, 3 (EPI 1, 2, 3) was an epidemiologic investigation of Schistosoma haematobium and/or S. mansoni infections in humans. The study methodology was designed and developed specifically to address the 3 EPI 1, 2, 3 objectives.

The epidemiology of schistosomiasis in Egypt: summary findings in nine governorates.

“…The data was stratified into 2 groups: 5 governorates in Lower Egypt (Kafr El Sheik, Gharbia, Menofia, Qalubia, and Ismailia) almost exclusively endemic for S. mansoni and 4 governorates in Upper Egypt (Fayoum, Minya, Assiut, and Qena) primarily endemic for S. haematobium ( Figure 1 of Hussein and others 6 ). Separation by geographic areas proved useful and credible since, except for the Fayoum, which had a 4.3% infection rate with S. mansoni, the amount of infection with the minor Schistosoma species in the other governorates was trivial (Ͻ2%, Table 1).…”

“…as low as 5% in villages or ezbas (satellite group of dwellings) with an 80% precision and 90% confidence level. 6 The findings are considered representative of the rural areas of the 9 governorates. The total sampled population was 120,907, but data on most variables were available on only 89,180 individuals: 49,950 in the Lower Egypt sample and another 39,230 from Upper Egypt.…”

“…6 Quantitative microscopic counting of Schistosoma ova in stool using a modified Kato technique and in urine using the Nucleopore (Pleasanton, CA) filter technique were performed as described. 8 Physical examination and abdominal ultrasonography were performed by trained physicians as described.…”

“…Under sponsorship of the Egyptian Ministry of Health and the United States Agency for International Development (USAID), the Schistosomiasis Research Project supported the investigation of prevalence and intensity of infection with Schistosoma sp., the prevalence and magnitude of morbidity caused by schistosomiasis, the changing pattern of distribution of S. mansoni and S. haematobium, and the determinants of infection and morbidity in a random sample of the rural inhabitants of 9 governorates in the country selected as representative of each area (Upper and Lower Egypt) and of governorates with both high and low infection rates ( Figure 1 of Hussein and others 6 ). This is the second national house-to-house survey for schistosomiasis in Egypt; J.…”

“…A. Scott's investigation was conducted 60 years previously. 6 Reported herein is a summarized version of the results from all 9 governorates.…”