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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 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
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?