Appendix 2.5 Overview of Survey

Methodology Key Populations Size Estimation

Session OverviewA. Why we need size estimatesB. Steps in conducting size estimatesC. Key methods for size estimation

1. Census and Enumeration2. Multiplier Method3. Capture-Recapture

Strengths and Limitations of each method

A. Data triangulation B. Recommendations and way forward

A. Why We Need Size Estimates of Key Populations (5)1. To understand about how many people there might

be (at higher risk for HIV transmission or infection) Important for planning and budgeting appropriate care and

prevention services

2. To project HIV disease burden and trends When combined with surveillance methods

3. To meet reporting needs that leverage resources for programs National government and Ministry of Health Global UNAIDS & other databases Basis for international contributions

And…4. To support policy decisions

Advocacy Existence and magnitude of risk populations; at local

and national levels For program planning and funds

Response planning Rationalize funding for interventions Calculating coverage of services

Efficient resource allocation Prevalence alone may distort allocation

And…5. To drive programming decisions

HIV services planning – prevention, treatment, and care Depends on number of people needing services at local

and national level

Scaling up Need size estimates to measure coverage

Monitoring and evaluation Size estimates can serve as a baseline to then assess

progress from (National and International targets; International

funding)

B. Steps in Conducting Size Estimates

General Area I

Preparation for conducting size estimates

Step 1 Determine the purpose

Step 2 Define the population and geographic area

Step 3 Determine the timing

Step 4 Review existing size estimates

General Area II

Consider the methods available

Step 5 Decide on the method

Step 6 Compile all existing data and collect additional data

General Area III

Analysis and dissemination

Step 7 Analyze and interpret the results

Step 8 Document the process

Step 9 Disseminate the results

Step 10 Use the data

Brainstorm (5 min) Which population should we prioritize?

How might we define the population?

Are there any existing size estimates of any Key Population?

C. Key Methods: Population Size Estimation

1. Census and Enumeration2. Multiplier methods3. Capture-Recapture

C.1. Census and EnumerationTheory

You can count The population can be counted You can make a map Population is concentrated and visible in distinct areas

Practicea) You make a map

b) Qualitative and quantitative information fills in the map: Existing data (programs, surveys) Focus groups Key informants Observation, field work

C.1.(a). Mapping The map is not the size estimate; mapping is a

process and tool

Mapping is essential to Census and Enumeration methods for size estimation

Mapping is useful (if not essential) to understanding the size estimates from other methods The geographic area Who is visible, who is not Who is included, who is not

Mapping is essential to program planning for Key Populations

Example of Mapping Police records of drug

arrests in a North American city

Where can you find people who inject drugs? Existing information

e.g., HIV cases Focus groups

Where else? Key informants

People who inject drugs, people who used to inject drugs

NGOs, services Gatekeepers Police

Observation in field Verify

C.1.(b). Qualitative and Quantitative Data

Census: The map guides a systematic count of everyone

The map forms a list of sites Count every member of the population at all sites

as rapidly as possible; add them up

Enumeration: The map is a sampling frame The map forms a list of sites; stratify sites by

characteristics e.g., brothels vs. street sites Select a random sample of sites by strata; count

population Apply averages to all sites by strata

Example of Census: MSM, Andhra Pradesh, India

No. District # Town

# of Spots

1 Srikakulam 15 48

2 Vizianagaram 7 20

3 Visakhaptnam 13 21

4 East Godavari 22 50

5 West Godavari 1 4

6 Nellore 14 50

7 Prakasham 9 20

8 Krishna 4 10

9 Guntur 14 80

  Total 99 303

Jan 2005

1 day Planning/TOT2 days Training of 120 participants/mappers3 days fieldwork2 days feedback/review of data

Source: Hindustan Latex Family Planning Promotion Trust (India)

Enumeration Method Example:Female Sex Workers Mapping of all entertainment centers in District A:

350 karaoke bars 276 massage parlors

A random sample is taken: 20 karaoke bars 20 massage parlors

Female sex worker counted at each, average per venue type: Karaoke bars: 6.7 Massage parlors: 2.4

Apply average to total for strata: 350 x 6.7 = 2,345 276 x 2.4 = 662 Total = 2,345 + 662 = 3,007 female sex workers

Census and Enumeration: Strengths & Limitations

Strengths Limitations

Understandable, straightforward

Conservative, lower limit estimates We saw them, we

counted them; must see at least this many

Reality check on other size estimation methods

Populations may not be visible

Census is time-consuming and expensive to conduct; Enumeration is somewhat less so

Both methods tend to underestimate

Brainstorm (5 min) Has anyone done Key Population-specific

mapping ?

Has anyone tried local Key Population-related census or enumeration?

Would we be able to count all members of a population?

C.2. Multiplier Method Multiplier Method

Uses two pieces of information about the same population and multiplies them to estimate the population size

Principles Get a count of the population seen at an institution (e.g.,

unduplicated number of Key Population seen at a service) in a specified time period: Good quality data that are unduplicated, and can

distinguish member from non-member of the population Implement a representative survey of the population

(e.g., BBSS) with specific question(s) “Did you go to <x facility> for <y service> during <z time

period>?”

Multiplier Method: Needs The two populations are equivalent, but independent

Being at the institution does not influence being in the survey or vice versa

The data sources are overlapping The population must have some chance of inclusion

in both sources The survey must be random and encompass the

population at the institution The institution data source must be specific to the

population being estimated The institution data must identify and distinguish the

population from other groups and align with target population of the survey

Multiplier Method: Example First data source (facility) should be specific to

the group being estimated If using STI clinic data to estimate size CSW

population, non sex-workers must be excluded from the list

Second data source (survey) should be random and encompass the group at the facility (CSWs); can include others as well Survey can include both brothel and street-based

sex workers

Multiplier–Related Methods Multiplier-like methods:

Unique object multiplier Distribute a memorable object to persons known to be

part of the population; in the survey, ask if they have the object

Unique event multiplier Conduct the survey some time after a population-

specific event (for which there is a list of unique attendees); ask if they attended the event

Capture-recapture (next!)

Unique Objects for Multiplier Examples

Unique Event Multiplier Method Example:

1,000 injection drug users were arrested in 2008 (count from police)

10% of injection drug users were arrested in 2008 (asked on a survey)

1,000 10% = 10,000

Size estimate: 10,000 injection drug users

Possible Sources of “Multipliers” Data Source Examples

STD, Social Hygiene Clinics Identified as [population x], registered

Police, incarceration (CSW. IDU) Arrested, registered, on probation

Entertainment center, brothel (CSW) Listed as employee

HIV care Clinic registries, surveillance cases Identified as [population x]

Prevention services Outreach contact for [population x],

Testing, laboratories HIVIdentified as [population x]

Unions, NGOs, social clubs Member on list of [population x],

Research Recent or on-going studies of [population x],

Multiplier MethodsStrengths and Limitations

Strengths Limitations Can use existing

data to produce [many] estimates on a national level

Straightforward if data sources are available

Flexible method useful in many circumstances

Need two independent data sources

Need high-quality data that is unduplicated

Needs consistent definitions between data sources: population, time periods, geographic catchment, age

Population members must all have a chance of being included in both data sources

Brainstorm (10 min) Related to the Multiplier Method, are you aware of

any possible data sources that could be used? Needs:

High quality Line listed Un-duplicated (if Al comes twice in a month, is he counted

just once?) Distinguishes the Key Population from all users

For which populations: MSM CSW Other Are there high quality line-listed person level data sources

available? Are they specific to the Key Populations?

C.3. Capture-Recapture Method Capture-Recapture

Collection of two (or more) sources of data on a population to estimate the missing data about the population Adapted from method used for size estimation of animal

populations in the wild

Direct Capture Conduct a “capture” and tag everyone before releasing;

conduct independent recapture and calculate proportion who were already tagged

Indirect Capture Use data from two institutions/services that a

population is in contact with

Capture-Recapture Method Principles

Two or more sources (lists, registries, observations) of cases

Sources considered independent samples from the same population

Cases can be matched by unique identifiers Estimates the total number of cases that are not

captured by any source from the matched and unmatched

Source: Crowcroft N. Health Protection Agency. London. Epi-Et class slides.

Capture-Recapture

Capture Recapture: Example Population definition: MSM who sell sex in and

around Mombasa, Kenya Method:

Capture 1: MSM peer leader enumerators distributed leaflets to

MSM identified as ‘on the market’ 284 leaflets distributed to MSM

Capture 2: One week later, 2nd sample captured at same places and

times 484 MSM sampled

Overlap (number of MSM sampled who also had a leaflet)= 186

(284 x 484) 186

Source: Geibel S et al. ‘Are you on the market?’: a capture-recapture enumeration of men who sell sex to men in and around Mombasa, Kenya. AIDS 2007; 21:1349-1354.

= Estimated 739 MSM who sell sex

Capture-Recapture Method:Strengths and Limitations

Strengths Limitations

Relatively easy to implement

Does not require much data

Does not require statistical expertise

Often not that simple in practice Difficult not to violate

assumptions Need a closed population (no

migration) Two samples must be

independent Must be able to identify people

accurately between samples The sample sizes of each

capture must be large enough to be meaningful

D. Data Triangulation Multiple size estimates for a particular

vulnerable population allow for cross-checks and validation

Present range (min. – max.) of estimates to reflect level of uncertainty inherent in the size estimation process

If independent results are in a similar range it increases confidence in the estimate

Triangulation of Multipliers for CSW, Suriname

7,166

2,343 2,305 2,196 2,000

1,4481158

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

STI-BSS Info-BSS 0.9%women

Condoms-BSS

Mapping Outreach-BSS

HIV test-BSS

Median: 2,196

95% CI: 1,726 – 2,666

Population Size of MSM in San Francisco by Diverse Methods, 2006

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

BRFS ACS Lieb-Case

Case-BSS

ATS-BSS

Case-HIS

CC-BSS

Trend CC-UMHS

Multiplier methods

Population based surveys Modeling

Med

ian

E. Recommendations and Way Forward Size estimation process should not be a ‘one

off’ activity but should rather be centrally managed by surveillance coordinating body

Methods that use already existing data should be prioritized

Multiplier methods have most promise in areas with good service data

Need to ensure that appropriate questions added to BBSS surveys

Need to strengthen institutional record-keeping

Brainstorm (10 min) Given that we are planning for a behavioural survey,

among Key Populations, could/should we include size estimation?

Based on your knowledge of the Key Population that you work with, how feasible will it be to utilise any or all of these methods to estimate the size of the population? Census and Enumeration

Multiplier Method

Capture-Recapture

How would you suggest it be implemented?

Thank You

Working Together to Plan, Implement, and Use

HIV Surveillance Systems

Some Materials Presented Here were Borrowed from the CDC/UCSF Training on Population Size Estimation held in the Caribbean Region in October, 2010.