Adv Epidemiology Ass't.pptx

Dambi-Dollo University
Institute of Medicine and Health Science
Department of epidemiology
Advanced Epidemiology
1
7/29/2022 Epidemiology Group 3
DADU

Sampling Approach’s to Hard-to-Reach Populations
DADU
2
Group Members
1 Mulatu Werkina……….. PGW/072/20
2 Eshetu Fikiru……………..PGW/050/20
3 Kiflu Kumera…………..PGW/069/20
4 Habib Chewaka……………PGW/063/20
5 Solomon Etefa……………..PGW/087/20

 Introduction
 Conventional cluster sampling
 Snowball or “chain referral” sampling
 Time-Location Sampling
 Respondent-driven Sampling (RDS)
 Indigenous field worker sampling (IFWS)
 Facility-based sampling (FBS)
 Targeted sampling (TS)
 Capture re-captures sampling method (CR) 3
DADU

Introduction
Hard-to-reach is a term used to describe those sub-groups of the
population that are difficult to reach or involve in research or
public health programmers’ due to their physical and
geographical location (e.g. in mountains, forests or deserts) or
their social and economic situation
Hard-to-reach populations are underground communities whose
members may be reluctant to self-identify and for whom no
sampling frame is available or can be constructed.
7/29/2022 Epidemiology Group 3 DADU 4

cont.
An alternative term “hidden population‟ is those who do not wish to be
found or contacted (e.g. illegal drug users or migrants and homeless
people)
This groups generally consist of sex workers, injection drug users, men
who have sex with men, and specific mobile or migrating population
groups.
These groups are difficult to identify and recruit due to their
marginalized status, desire for anonymity, stigma associated with their identities
or behaviors, and/or fear of legal repercussions

cont.
• There are other subgroups for which sampling methods for “hard-to-
reach” populations can be applicable.
Those are tourists, adolescents with limited access to healthcare, and gig-economy
workers).
While such populations do not experience the same social marginalization
as traditional hard-to-reach groups, sampling frames are usually
unavailable.
 For example, though companies have rosters of gig workers, they may be reluctant
to share these rosters with researchers.

People who are sometimes categorized as being hard-to-reach or
hidden
• Those being under social pressure of the broader community
• Those living in faith based communities
• Those who fear of confrontation with legal authorities
• Illiterates
• Those who have no interest to be found or contacted
• Migrants
• Newly arrived residents
• Over-researched people
• Those living in remote physical and geographical location
• Those living in vulnerable social and economic situation
7
7/29/2022 Epidemiology Group 3 DADU

cont…
• Different sampling techniques were introduced so far to recruit hard-to-
reach populations.
• Sampling methods are broadly categorized as either probability-based
methods or non-probability-based methods.
• in probability sampling, the probability any given individual in the
population is included in the sample is known or can be estimated using
information about the sampling process.
8

cont…
Common methods for sampling hard-to-reach populations
include
non-probability-based approaches (e.g., convenience cluster
sampling, snowball sampling) and
probability-based approaches (e.g., time-location sampling,
respondent driven sampling).
9

1 Conventional cluster sampling
 can be applied in limited circumstances to recruit hard-to-reach
population.
 is reasonable when there is no list of people to be selected, but a good list
of locations where individuals from hard-to-reach group are gathered.
The primary presumption for use of this sampling method is that the
distribution of the variable of interest is similar between locations.
10

cont…
• Clusters then are randomly selected for data gathering and thus
recruitment costs will be minimized since the number of locations from
which recruitment take place reduces.
• Clusters can be perinatal clinics, drug treatment centers, restaurants or
health centers.
• Other requirement to be met in using this method is the possibility of
ready access to all individuals from the population of interest in the
clusters;
 otherwise cluster sampling will be an infeasible option to reach hidden
populations
11

Cont.
• For cluster sampling to be appropriate, it is necessary to have available
or be possible to construct a relatively complete sampling frame of
group members.
• Furthermore, it is necessary to be able to access all group members
during the period of data collection.
• These requirements might be met in the case of readily accessible
populations, for example, military personnel and miners.
12

Cont.
Other groups of potential interest (e.g. police, transportation workers),
unless it is possible to make repeated ‘callbacks’to obtain
measurements from sampled group members not present at the time of
data collection, could result in a potentially large non-response bias,
rendering cluster sampling an infeasible option for hidden high-risk
groups.
13

2. Snowball or “chain referral” sampling.
• Snowball sampling is a non-probability method.
• A study sample based on referrals made among people who share or
know of others who possess some characteristics that are of research
interest.
• Primarily used in sociologic (qualitative) research
• Allows for sampling of natural interactional units
14

Cont.
• Identify participants (opinion leaders or key-informants) to start the chain
referral; this is easily done with outreach workers or other study participants.
• Can accept participants who are part of a chain and those who are not.
• Important to keep track of index person who made the referral for later
analysis on degree of clustering.
• May or may not want to divulge eligibility criteria to index persons.
15

Cont.
• used when the desired sample characteristic is rare or
• when the studied population is broader and more heterogeneous
than that can be easily accessible through other more reliable
sampling methods.
• In the snowball sampling method reliance is based on referrals
from initial known subjects to recruit new additional subjects.
16

Cont.
• This method is often used when the population under investigation is hard-
to-reach due to their special characteristics or sensitivity of the study
subject.
• The known cases may be contacted to acquire needed data.
• If these known cases mutually agree to participate in the study they will be
asked to nominate and facilitate introductions to other people whom they
know according to the interpersonal relations and connections between
people.
17

cont…
• Accordingly, the introduced nominee will be contacted and invited to
participate in the study and with consent he or she is asked to introduce
other people who also fulfill the study inclusion criteria.
• This strategy is regarded as a potential solution to the problem of
sampling concealed populations.
18

cont…
• The basic assumption in snowball sampling is that a link exists
between the initial known subjects and others in the same
target population.
• If this assumption is accurate it will allow a chain of
acquaintance to be created.
19

Control the Types of Chains and Number of Cases in
each Chain
• Substantive consideration: Once chains begin, an investigator may want
to control the referral chains to help ensure that the general
characteristics of the target population are reflected in the sample.
• Selective consideration: May want to exclude or limit a certain subgroup
(i.e., certain age group)
• Theoretical consideration: What are some logical assumptions that can be
made about certain groups (i.e., certain neighborhoods may use certain
types of drugs and drugs purity may differ, etc.)
20

3 Time Location Sampling
• TLS is known as time space sampling and venue based sampling used to
collect data from hard to reach populations, such as MSM, who can be
found at identifiable locations.
• bias due to masking and chain-referral selection of study participants is
eliminated in TLS .
• In order to use the TLS method with MSM, FSW and truck drivers, it
was necessary to first construct the sampling frame for each population
in every city.
21

Cont.
• Using structured interviews with key persons, lists of venues
used by these populations were drawn up with geographical
indications.
• The physical existence of these venues was verified, as well as
their opening and closing times, and an estimate of the size of
the target population present
22

Cont.
 The ‘time-location’units were established, a random selection took
place, and quotas were set for interviews at each location, taking into
account the estimated population size in each venue.
 The conglomerates were represented by the venues identified for each
population.
 For example, for male and female sex workers, these were formed by
bars, strip clubs, meeting places in specific streets or sections of beach,
etc.
23

Cont.
• For long distance truck drivers, the venues were car parks, cafes and
restaurants for truck drivers, transport companies, and resting places
along motorways.
• for people experiencing homelessness may include homeless shelters,
public parks, specific city blocks, and facilities that provide services
• Once the location had been chosen on one day of the week, and for a
specific time, the members of the target population to be interviewed
were randomly selected if the number of potential participants exceeded
that set for the quota.
24

cont.
If the number of people from the target population was lower
than that established in the quota on the first visit to the venue,
the visit was repeated the following week, respecting the day of
the week and the time established through random selection.
25

Strengths and Limitations of TLS
Strength
• Assumed representativeness by approximating random cluster sampling
• Efficient for rare or hard to reach populations
• No need for a complete roster of individuals
Limitations
• • need complete “map” of venue day-times
• somewhat hard to validate
• bias towards those who attend venues, leaves out those who never or
rarely attend
26

4 Respondent-driven Sampling (RDS)
• RDS is a peer-referral probability-based sampling method developed in
1997 by Douglas Heckathorn via a study of AIDS prevention among
people who inject drugs.
• A type a chain referral sampling which involves an incentive system.
• Reduces some of the known biases of other chain referral methods.
• Provides a means for controlling for the biases resulting from
differences in the size of personal networks.
27

cont…
• A researcher approaches and enrolls a participant who
receives an incentive for undergoing the study interview.
• Same participant is also given an incentive for bringing in
their peers as respondents.
RDS sampling consists of the following three steps to facilitate
recruitment procedure
28

cont…
1 Seed selection. All RDS studies begin with a small number of seeds from the target
population (e.g. 3-15). Seeds should be diverse and networked, but they don’t need to be
choose randomly
2 Interviews and recruitment: seeds complete the interview process and receive a
predetermined number of coupons that they can use to recruit other people like them wave 1.
The recruits of wave 1 then complete the interview process and recruit wave 2. this referral
chain continues until the desired sample size is reached.
3 Incentive: participants receive two incentive: one for completing the interview, and one for
each peer that is successfully recruited.
29

Respondent-driven sampling checklist
The RDS sampling method includes four essential elements.
If one or more of these is not present, then the sampling method is not RDS.
These are:
 i documentation of who recruited whom must be tracked, generally through a coupon
system;
 ii recruitment must be rationed with generally no more than three coupons allotted per
‘seed’,
 iii information on personal network size must be gathered and recorded; and
 iv recruiters and recruits must know one another (i.e. have a preexisting relationship).
30

5 Indigenous field worker sampling (IFWS)
• In this sampling method instead of using formal trained investigators, they are
selected from local community.
• Then they undergo special training relevant with objectives of the study including
interview skills and fieldwork protocol.
• The selected people should have privileged access to the study target population.
• It is believed that use of this technique can reduce masking, volunteer bias and
under-reporting of socially undesirable behaviors.
31

cont…
 The indigenous fieldworkers track down individuals known to them within the target
area and recruit them into the study.
 Data collection takes place in the community setting separate from the rest of
research team.
 An incentive is given to participants and they are asked to introduce their peers to
the interviewer.
 To ensure wide coverage of the target population use of multiple sites and
recruitment networks is recommended
32

6 Facility-based sampling (FBS)
Facility-based sampling refers to recruiting members of target population
from a variety of facilities including correctional and drug treatment centers,
sexually transmitted diseases clinics or general health centers and hospitals
in certain suburban areas .
Each of these facilities can be used to recruit individuals from hidden
population, but similar biases may occur due to under-sampling of those who
are reluctant to seek and obtain services especially when their behaviors are
stigmatized.
33

cont…
limitations of this sampling method include that in many part
of the world, particularly in less developed countries,
dedicated services to high risk groups are not common and
even where provided; equal access to them by deprived
subgroups of population is not guaranteed.
34

7 Targeted sampling (TS)
The targeted or purposive sampling method has been developed to
overcome the limitation of snowball sampling when we would like to
include specific pre-defined subgroups of population in our sample.
This sampling method generally includes an initial assessment aimed at
identifying the various subgroups that might exist in the population of
interest.
35

cont…
• The identified subgroups are then regarded as sampling strata, which
should have a pre-defined quota in the final sample.
• The magnitude of success in this sampling method depends mainly on
thoroughness of the initial assessment and to some extent the time and
resources available for its undertaking.
• Application of this method can reach readily accessible subgroups very
quickly, but on the other hand reaching isolated people will be very time
consuming and therefore expensive.
36

Cont.
• Potential Biases in Targeted Sampling: Recruiting participants
during normal business hours, Activities focused on more
public locations (“institutional” bias may occur).
37

8 Capture re-captures sampling method (CR)
A technique used to estimate the size of populations that are difficult to
find and count, or populations that are in motion and cannot be counted
all at one time.
Technique requires obtaining two or more independent observation on
the same populations at the same times or based on different sources
that represent the same population.
38

cont.
The researcher needs to know three things:
1 number of persons observed at the first time
2 number of persons observed at the second time
3 Total number of persons observed at both times
39

cont…
Those captured must be “tagged” in some way so that it may be
determined if they were included both captures.
In employing this method, the primary assumption is equal probability
of being sampled during the study period, different individuals can be
missed in the counting process for various reasons.
40

cont…
• Assumptions:
– Equal probability of being sampled during the study period
• Different individuals can be missed in the counting process for various reasons.
– Independence between counts: observing an individual at one time has no effect on the event of
observing the individual at a second.
• Being missed once can often be correlated with being missed a second time.
– Population size does not change in composition or size during the study period (“closed
population model”).
• Population being studied changes between captures; “open population model”; need at least
four captures.
41

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