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Module 4 Survey design TAS Global Programme to Eliminate Lymphatic Filariasis (GPELF) Training in monitoring and epidemiological assessment of mass drug administration for eliminating lymphatic filariasis Module 4 Survey design
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Module 4 Survey design

Feb 24, 2016

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Training in monitoring and epidemiological assessment of mass drug administration for eliminating lymphatic filariasis. Module 4 Survey design. Learning objectives. By the end of this module, you should understand how to determine: a survey site the sampling strategy the sample size - PowerPoint PPT Presentation
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Page 1: Module 4 Survey design

Module 4 Survey design

TAS

Global Programme to Eliminate Lymphatic Filariasis (GPELF)

Training in monitoring and epidemiological assessment of mass drug administration for eliminating lymphatic filariasis

Module 4 Survey design

Page 2: Module 4 Survey design

Module 4 Survey design

Learning objectives

By the end of this module, you should understand how to determine:

1. a survey site

2. the sampling strategy

3. the sample size

4. a critical cut-off

Slide 2

Page 3: Module 4 Survey design

Module 4 Survey design

Overview

Target population Survey site Sampling strategy

Cluster sampling

Systematic sampling

Census

Sample size Critical cut-off

Slide 3

Page 4: Module 4 Survey design

Module 4 Survey design

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.

Slide 4

Cluster-based sampling Systematic sampling Census

School Community

Sample size and critical cut-off

Evaluation unit1. Survey area

2. Survey site

3. Sampling strategy

4. Sample size

Page 5: Module 4 Survey design

Module 4 Survey design

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.

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.

Slide 5

Page 6: Module 4 Survey design

Module 4 Survey design

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.

Survey site

Options: School-based survey Community-based household survey

Slide 6

A TAS can be conducted in schools or in communities, depending on the proportion of 6- and 7-year-old children in schools.

Page 7: Module 4 Survey design

Module 4 Survey design

Options: Cluster sampling

o Cluster = sampling unit = school or enumeration area (smallest area for which census results are available, e.g. village or ward)

o Select clusters, then systematically test only children in selected clusters

o Advantage: fewer sites to visit Systematic sampling

o Sample at all siteso Select children to test at fixed intervalso Advantage: smaller sample

Census o No sampling required; test all children in target age range at all sites

Sampling strategy

Slide 7

Page 8: Module 4 Survey design

Module 4 Survey designSlide 8

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)

Page 9: Module 4 Survey design

Module 4 Survey designSlide 9

Cluster sampling

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

Often used when the population is large or there are many schools or enumeration areas

Page 10: Module 4 Survey design

Module 4 Survey designSlide 10

Systematic samplingOften 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

Page 11: Module 4 Survey design

Module 4 Survey designSlide 11

CensusOften 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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Module 4 Survey design

Algorithm for survey site and sampling strategy

Slide 12

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Module 4 Survey design

Sample size

Slide 13

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.

Page 14: Module 4 Survey design

Module 4 Survey design

Sample size

Slide 14

24 000 1 556

Page 15: Module 4 Survey design

Module 4 Survey design

Critical cut-off

Slide 15

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.

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.

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Module 4 Survey design

Critical cut-off

Slide 16

24 000 1 556 18

Page 17: Module 4 Survey design

Module 4 Survey design

Critical cut-off in census

Slide 17

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%

Page 18: Module 4 Survey design

Module 4 Survey design

Exercise

Slide 18

1. Using Figure 3 on p. 25 of the 2011 WHO monitoring and evaluation manual:i. determine whether a school-based or a community-based

survey is appropriate for the EU(s) defined in module 3.

ii. determine whether a cluster, systematic or census sampling design is appropriate.

2. Using Table A.5.1or A.5.2 on pp. 73–74 of the 2011 WHO monitoring and evaluation manual:i. determine the sample size needed for the EU(s) defined in

module 3.

ii. determine the critical cut-off for the survey(s).