Appendix 2.1:RDS and TLSAn Overview of Probability-Based Sampling

Methods For Key Populations

Session Overview Discussion of Sampling Hard to Reach

Populations Presentation, comparison, and discussion of:

Time Location Sampling (TLS) Respondent Driven Sampling (RDS)

How to Reach Hard to Reach Populations ….. Let’s Discuss!

What Makes RDS and TLS Unique for Sampling Key Populations?

They are probability-based sampling methods

Every respondent has a non-zero, known probability of being selected to participate in the study

With weighting, the sample can be made representative of the target population Characteristics of the sample are valid estimates

of the characteristics of the target population

Time Location Sampling (TLS) The method is known by several names

Venue Day Time (VDT) sampling Temporal-spatial sampling (TSS) Time Venue Sampling (TVS) Variation of Targeted Sampling (TS) Venue-Based Sampling (VBS)

The idea is to sample the target population at randomly selected venues where members of the population are known to congregate

Basic Principles of TLS Approximates random cluster sampling

1. Target population is divided into clusters or groups2. Groups of individuals are randomly selected for

sampling3. Individuals in selected groups are randomly

sampled

In TLS, the sampling clusters are known as venue-day-times (VDTs). VDTs are the: Places Day of the Week And times where target population can be found

Basic Steps for TLS1. Identify the universe of venues-day-time

(VDT) periods attended by the target population

2. Build a sampling frame of VDTs3. Randomly select VDTs for recruitment events4. Systematic Sampling: Intercept, consent,

interview, and VCT at event5. Data management6. Analysis, interpretation7. Use the data!

Venues-Day-Time (Cluster Sampling Frame)

Name Address

Phone

Mon Tues Wed Thurs Fri Sat Sun

Power Fitness

2230 18th St

6-10pm

6-10pm

West End Video

1984 West End Place

234-2390

8pm-12am

8pm-12am

8pm-12am

8pm-12am

Café Noir

28 Sheppard

834-4823

6-10pm

6-10pm 6-10 pm 6-10 pm10pm-12am

6-10 pm10 pm-12am

4 -8pm

Men’s Choir

1438 Oak St.

N/A 8-9pm

Creating a complete universe of venues-day-times is a lot of work!

Assumptions of TLS “Map universe” of all the places where and

when the target population can be found Randomly sampling enough places and times

provides everyone in the target population equal chance of being in study

If not equal chance, there are methods to “weight” sample according to who are more or less likely to be at the venues

Ethical Challenges in TLS Ensuring anonymity Returning results, ensuring appropriate care Drawing unwanted attention to safe havens

(police, public) Intoxicated participants Balancing random sample with outreach and

prevention

Chain-Referral (Snowball) Sampling Recruitment through a network

Participants recruit individuals from their personal network to participate

Strength: Reaches respondents who avoid public venues and

institutions May have greater coverage because respondents are

reached through their social networks Weakness:

This is a convenience sample rather than a probability sample

Characteristics of the sample are NOT valid estimates of the characteristics of the target population

Respondent-Driven Sampling (RDS) RDS combines sampling coverage and

probability sampling methods Combines a modified form of chain-referral with a

mathematical system for weighting the sample to compensate for its not having been drawn randomly

Based on the premise that peers are better able than outreach workers and researchers to locate and recruit other members of a hidden population

Unlike other chain referral methods, gives valid population point estimates with standard errors

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

Controlled Recruitment = Penetration / Sample Size Growth

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

Controlled Recruitment = Penetration / Sample Size Growth

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

Controlled Recruitment = Penetration / Sample Size Growth

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

Controlled Recruitment = Penetration / Sample Size Growth

Controlled Recruitment = Penetration / Sample Size Growth

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

Controlled Recruitment = Penetration / Sample Size Growth

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

RDS WeightingRDS has two types of weighting: Recruitment pattern weighting

Used to adjust for differential recruitment between groups

Network size weighting Well-connected individuals tend to be over-

sampled because many recruitment paths lead to them

Requirements of RDS Four requirements:

Document who recruited whom Recruiter and recruit must know one another Ration recruitment so a few cannot do all the

recruiting (i.e., three recruits/recruiter) Must ask recruiter and recruit about personal

network sizes

If a study does not include these features, it is not RDS

Steps in RDSIdentify, Recruit

and Interview Seeds

Provide seeds info on who and how to

recruit

Give 3 - 5 coupons to each seed

Recruits bring valid coupons to the study site; If eligible they

are recruited

Every new recruit is then

asked and given 3 - 5 coupons

The recruiter is rewarded for every coupon

redeemed

“Coupon Manager” tracks coupons/links recruiters and peers

RDS Assumption (Heckathorn)1. Individuals know each other as members of

the population in question2. Target population forms one single large

network3. Report network size accurately4. Recruit randomly from network5. Sampling with replacement

Advantages of RDS Maintains privacy of population Team can be centrally located Less logistical needs Ease of field operations Moderate formative research/mapping Target members recruit for you Reach less visible segments of population Good external validity Minimal number of additional questions needed Computer software available Generally lower cost

Challenges of RDS Coupons can be slow in being redeemed Population must be networked Must be able to verify group membership Must track links between recruiters and

recruits-coupon management Appropriate incentive levels Very difficult to deal with selective non

response bias Analysis is complex

Breakout (20 minutes): TLS vs. RDSWhich methodology might be more appropriate for the

target population here in the Bahamas?a) Do individuals in the target population frequent venues?b) Could the target population be identified at these venues?c) Could the target population be accessed at these venues

without drawing them unwanted attention?d) Might we miss some population groups at these venues?e) Are individuals in the target population well-networked?f) Are there distinct sub-populations?g) Do sub-populations interact with each other?h) Can members of the target population identify each

other?i) Will the target population be motivated to participate?

Thank You

Working Together to Plan, Implement, and Use

HIV Surveillance Systems