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Training in monitoring and epidemiological assessment of mass drug administration for eliminating lymphatic filariasis Module 4 Survey design

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Learning objectives By the end of this module, you should understand how to determine: a survey site the sampling strategy the sample size a critical cut-off

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**Overview Target population Survey site Sampling strategy Sample size**

Cluster sampling Systematic sampling Census Sample size Critical cut-off

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**Determining survey site, sampling strategy and sample size**

Once the survey area (EU) has been defined, the next steps are to determine the survey site, sampling strategy and sample size. 1. Survey area Evaluation unit 2. Survey site School Community 3. Sampling strategy Cluster-based sampling Systematic sampling Census 4. Sample size Sample size and critical cut-off

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**Target population Target group: Children aged 6–7 years**

Why?: Young children should have been protected from infection if MDA was successful in interrupting transmission. Positive test results in this age group therefore usually indicate recent transmission. For school-based surveys: All children enrolled in selected grades (usually grades 1 and 2) should be considered eligible for the survey sample. For community-based household surveys: All children aged 6–7 years in the EU are eligible for inclusion.

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Survey site A TAS can be conducted in schools or in communities, depending on the proportion of 6- and 7-year-old children in schools. Options: School-based survey Community-based household survey If the net primary-school enrolment ratio in the EU is ≥ 75%, the survey can be conducted in schools. The net school enrolment ratio should be confirmed with the ministry of education. The enrolment ratios for the EU should be used, if available. Good judgement should be used if the rates in the EU vary. If the net primary-school enrolment ratio is < 75%, a community-based household survey should be conducted.

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**Sampling strategy Options: Cluster sampling**

Cluster = sampling unit = school or enumeration area (smallest area for which census results are available, e.g. village or ward) Select clusters, then systematically test only children in selected clusters Advantage: fewer sites to visit Systematic sampling Sample at all sites Select children to test at fixed intervals Advantage: smaller sample Census No sampling required; test all children in target age range at all sites

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Sampling strategy Choice between cluster and systematic sampling depends on: the total number of children in the target age range (6–7 years) the total number of clusters (i.e. schools or enumeration areas) in the EU A census should be conducted in areas where the total target population is small (i.e. < 400 children in areas where Anopheles or Culex is the principal vector; < 1000 children in areas where Aedes is the principal vector)

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Cluster sampling Often used when the population is large or there are many schools or enumeration areas Step 1: Randomly select clusters (schools or enumeration areas) to be visited Step 2: Randomly select children to be tested only within each selected cluster

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Systematic sampling Often used when the population is small to medium or if there are fewer than 40 schools or enumeration areas Step 1: Visit all schools or enumeration areas Step 2: Randomly select children to be tested in each school or enumeration area

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**Census Often used when the population is small**

< 400 children of target age in areas where Anopheles or Culex is the principal vector < 1000 children of target age in areas where Aedes is the principal vector Step 1: Visit all schools or enumeration areas Step 2: Test all children

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**Algorithm for survey site and sampling strategy**

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Sample size Options: Table A.5.1 or A.5.2 in Annex 5 of the 2011 WHO monitoring and evaluation manual (pp. 73–74) Survey sample builder Sample size depends on the total population of target-age children in the EU and the sampling method used. Because Aedes spp. are more efficient vectors, the target level of antigenaemia is lower in these areas. As a result, the sample sizes will be larger than in areas where Anopheles, Culex or Mansonia is the vector.

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Sample size 24 000 1 556

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Critical cut-off Critical cut-off: Threshold of infection prevalence below which transmission is assumed to be no longer sustainable, even in the absence of MDA. TAS provides an estimate of this threshold in the EU as the number of antigen- or antibody-positive cases. If the number of positive cases is at or below the established cut-off, the EU ‘passes’, and governments can decide to stop MDA. If the number of positive cases is above the established cut-off, at least two more rounds of MDA should be conducted.

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Critical cut-off 24 000 1 556 18

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**Critical cut-off in census**

In areas where a census is conducted, a point prevalence of infection is calculated. MDA can be stopped in: areas of transmission by Culex, Anopheles or Mansonia in which the prevalence is < 2% areas of transmission by Aedes in which the prevalence is <1%

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Exercise Using Figure 3 on p. 25 of the 2011 WHO monitoring and evaluation manual: determine whether a school-based or a community-based survey is appropriate for the EU(s) defined in module 3. determine whether a cluster, systematic or census sampling design is appropriate. Using Table A.5.1or A.5.2 on pp. 73–74 of the WHO monitoring and evaluation manual: determine the sample size needed for the EU(s) defined in module 3. determine the critical cut-off for the survey(s).

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