estimating hard-to-reach populations with network sampling...vincent, k. (2018). recent advances on...
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Estimating Hard-to-Reach Populations withNetwork Sampling
Kyle Vincent
Network Sampling NAS Presentation
Outline
Overview of Contemporary Network-Based Strategies
Novel Strategy
Advantages and Disadvantages
Practice
Final Thoughts
Network Sampling NAS Presentation
What is network sampling?
A referral-based strategy for sampling individuals
Also know as link-tracing, peer-referral
Trace social links to enlarge sample to sufficient size
Strategy can overcome limitations presented withconventional approaches
Network Sampling NAS Presentation
What is network sampling?
Simulated population: N = 147
Network Sampling NAS Presentation
What is network sampling?
Initial sample: n0 = 10
Network Sampling NAS Presentation
What is network sampling?
First wave: n1 = 10
Network Sampling NAS Presentation
What is network sampling?
Second wave: n2 = 10
Network Sampling NAS Presentation
Contemporary Network-Based Strategies
Snowball Sampling
Respondent Driven Sampling (RDS)
Network Scale Up Method (NSUM)
Network Sampling NAS Presentation
Novel Strategy
Motivation is based on:
1) Need for estimate for population size
2) Conduct sophisticated network analyses
Network Sampling NAS Presentation
Novel Strategy
Exploit available resources to select a representative initialsample (need help from partnering agencies, NGOs,service provider, etc.)
Recent work for stratified setup can help with this (Vincent,2018) - relies on mark-recapture theory
Network Sampling NAS Presentation
Novel Strategy
Base preliminary estimates on information corresponding toinitial sample selections (Vincent, 2018).
Consider count of links from initial sample to outside initialsample
Consider count of links within initial sample
Relative measure of density of links within sample
Network Sampling NAS Presentation
Novel Strategy
Base improved estimates on strategy presented in Vincent andThompson (2016), Vincent (2018).
Computationally intensive procedure
Enumerate all sample reorderings, (A,B,C), (B,A,C),(C,A,B), ...
Network Sampling NAS Presentation
Novel Strategy
Colorado Springs Study; Klovdahl et al. (1994). N = 595.
Network Sampling NAS Presentation
Novel Strategy
n0 = 53, N̂ = 758.
Network Sampling NAS Presentation
Novel Strategy
n0 = 85. N̂ = 568. Substantial improvements with link-tracing(Vincent and Thompson, (2016) and Vincent (2018)).
Network Sampling NAS Presentation
Advantages
No need to sample multiple waves
Sampling multiple waves does not add any bias toestimators
Can give estimators based on each wave
Network Sampling NAS Presentation
Advantages
Strategy can be applied in a spatial setting.
Network Sampling NAS Presentation
Advantages
Strategy can be applied in a spatial setting.
Network Sampling NAS Presentation
Advantages
Strategy can be applied in a spatial setting.
Network Sampling NAS Presentation
Advantages
Strategy can be applied in a spatial setting.
Network Sampling NAS Presentation
Advantages
Can augment most network samples with other samples toget some valid MSE/mark-recapture estimates
Gives rise to a rich data set, good for social networkanalyses
Gain some understanding of spatial relations, networktopology
Network Sampling NAS Presentation
Advantages
Ability to adapt/steer link-tracing to units of higher-interest
Also know as adaptive sampling
Gives a higher yield of interesting units in the final sample
Network Sampling NAS Presentation
Adaptive Sampling
Simulated population: N = 147
Network Sampling NAS Presentation
Adaptive Sampling
Initial sample: n0 = 10
Network Sampling NAS Presentation
Adaptive Sampling
First wave:
Network Sampling NAS Presentation
Adaptive Sampling
Second wave:
Network Sampling NAS Presentation
Disadvantages
Requires a good start, i.e. initial sample, need help frompartnering agencies, NGOs, service provider, etc.
Need to know nominations within final sample, link withinsample (and across samples, like in MSE/mark-recaptureapproaches)
Network Sampling NAS Presentation
Practice
International Labour Organization (ILO)
ILO sought a global estimate on modern slavery
Data sources include media reports, governmentdocuments, academic reports, ILO reports, etc., mostlyaccessible online
Idea: conceptualize data sources as network population
Network Sampling NAS Presentation
Practice
Sources are nodes, directed links between nodes if onereferences the other
Apply novel strategy to this population
See Zhang and Vincent (2017) for paper
Network Sampling NAS Presentation
Practice
International Justice Mission (IJM)
Study on trafficked sex workers attached to public andprivate establishments in hotspots of Kolkata and Mumbai,India
Used a link-tracing sampling design
Estimates of population size presented in Parks et al.(2017)
Network Sampling NAS Presentation
Practice
Enlarged nodes represent contacts, green for commercialsex-workers, red for pimps/madams, yellow for other.
Network Sampling NAS Presentation
Final Thoughts
The new strategy:
Can provide efficient estimates of population size, rich dataset with practical design
Facilitate intervention strategies, since these are typicallyachieved through a link-tracing design (Thompson, 2017)
Network Sampling NAS Presentation
References
Klovdahl, A., Potterat, J., Woodhouse, D., Muth, J., Muth,S., and Darrow, W. (1994). Social networks and infectiousdisease: The Colorado Springs Study. Social Science &Medicine 38, 79-88.
Parks, A., Vincent, K., Nie, Z., Russel, A. (2017). A studyof public and private establishment-based commercialsexual exploitation in Kolkata and Mumbai, India. Inpublishing phase.
Thompson, S. (2017). Adaptive and network sampling forinference and interventions in changing populations.Journal of Survey Statistics and Methodology 5, 1-21.
Network Sampling NAS Presentation
References
Vincent, K. and Thompson, S. (2016). Estimatingpopulation size with link-tracing sampling. Journal of theAmerican Statistical Association. Accepted for publication.
Vincent, K. (2018). Recent advances on estimatingpopulation size with link-tracing sampling. Submitted.
Zhang, S. and Vincent, K. (2016). Strategies to estimateglobal prevalence of trafficking for sexual exploitation.CHANCE. Accepted for publication.
Network Sampling NAS Presentation