Appendix 2.1:RDS and TLS An Overview of Probability-Based Sampling Methods For Key Populations

1. Appendix 2.1:RDS and TLSAn Overview of Probability-Based Sampling Methods For Key Populations

2. Session Overview • Discussion of Sampling Hard to Reach Populations • Presentation, comparison, and discussion of: • Time Location Sampling (TLS) • Respondent Driven Sampling (RDS)

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

4. 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

5. 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

6. Basic Principles of TLS • Approximates random cluster sampling • Target population is divided into clusters or groups • Groups of individuals are randomly selected for sampling • 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

7. Basic Steps for TLS • Identify the universe of venues-day-time (VDT) periods attended by the target population • Build a sampling frame of VDTs • Randomly select VDTs for recruitment events • Systematic Sampling: Intercept, consent, interview, and VCT at event • Data management • Analysis, interpretation • Use the data!

8. Venues-Day-Time (Cluster Sampling Frame) Creating a complete universe of venues-day-times is a lot of work!

9. 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

10. 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

11. 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

12. 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

13. Controlled Recruitment = Penetration / Sample Size Growth Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

14. Controlled Recruitment = Penetration / Sample Size Growth Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

15. Controlled Recruitment = Penetration / Sample Size Growth Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

16. Controlled Recruitment = Penetration / Sample Size Growth Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

17. Controlled Recruitment = Penetration / Sample Size Growth Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

18. Controlled Recruitment = Penetration / Sample Size Growth Wave 1 Wave 2 Wave 3 Wave 4 Wave 5

19. RDS Weighting RDS 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

20. 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

21. Steps in RDS Identify, 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

22. RDS Assumption (Heckathorn) • Individuals know each other as members of the population in question • Target population forms one single large network • Report network size accurately • Recruit randomly from network • Sampling with replacement

23. 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

24. 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

25. Breakout (20 minutes): TLS vs. RDS Which methodology might be more appropriate for the target population here in the Bahamas? • Do individuals in the target population frequent venues? • Could the target population be identified at these venues? • Could the target population be accessed at these venues without drawing them unwanted attention? • Might we miss some population groups at these venues? • Are individuals in the target population well-networked? • Are there distinct sub-populations? • Do sub-populations interact with each other? • Can members of the target population identify each other? • Will the target population be motivated to participate?

26. Thank You Working Together to Plan, Implement, and Use HIV Surveillance Systems