Foundations of
Research
11
Research Sampling.
 What is the target
population for your study?
 Sampling:
 Probability
 Non-Probability
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Foundations of
Research
Research Sampling.
 Define your target population

 What group do you want to generalize to?
 What is your sampling frame?
 Who is / is not a member of the group?
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Foundations of
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
The target population
Any study assesses only a sample of the population.

We always must generalize from our sample to the larger
population.


Research often addresses a specific population or subpopulation.


Even the census does not enroll 100% of Americans.
Our definition of the population to target is an important step.
The size and breadth of the population we are generalizing
to can affect the Internal or External validity of the study.
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Foundations of
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The overall flow of sampling decisions.
We begin with a decision
about who we are
interested in.
We then make decisions about who
is in the target population and how
to recruit them.
From there we collect our sample,
representing a very small % of the
larger population.
We get results of our experiment or
study within our sample.
General population or
targeted sub-population
Sampling frame
The sample
Inference
Study results
…and attempt to infer what the entire target population
must be like.
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Foundations of
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Sampling; the population
We may design our study to
inform us about a very
general population.
So, a cognitive-neuroscience study
may test hypotheses about the
brain generally.
General population or
targeted sub-population
Sampling frame
Often we study a subgroup, e.g.
women, a class of medical patients,
the homeless...
The sample
Many studies compare segments of
Inference
the general population, e.g.,
African-Americans & Caucasians.
Study results
This more narrow focus makes
generalization more clear.
Psychology studies purport to generalize broadly, but by enrolling
only college students or survey volunteers actually generalize to a
very small portion of the population.
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Research
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The Sampling Frame
Sampling frame refers to
several elements of our
study;
General population or
targeted sub-population
• What do we already know about
our population?
 Do we have census or other
Sampling frame
data?
• What criteria do we use to
determine who is or is not a
member of a target population?

If we are studying
homelessness, who “counts” as
homeless?
The sample
Inference
Study results
• How do we contact and recruit participants?

Where may a representative sample of our target population be reached?

How do we actually approach and enroll them?
Foundations of
Research
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Sampling; assignment
There are many ways for us
to sample our target
population(s).
The main distinction is Probability
vs. Non-Probability sampling. We
will address that later.
Within our sample we may use
Blocking Variables to compare
different segments, e.g., age or
ethnic groups.
For experiments we assign
participants to groups, typically
using Random Assignment.
General population or
targeted sub-population
Sampling frame
The sample
Inference
Study results
Randomized Block Assignment may, for example, randomize within
ethnic blocks, to ensure that the same proportion of African-Americans,
Caucasians and Latinos are in each group.
Foundations of
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Sampling: population inferences
In Inferential Research we
are not interested in simply
describing or analyzing our
sample.
We use our results to Infer the
characteristics of the larger
population we sampled from.
The quality of our inference is
shaped by factors such as how
Reliable and Valid our measures
are.
General population or
targeted sub-population
Sampling frame
The sample
Inference
From a sampling perspective,
Study results
Statistical Power is a key element.
Power refers to whether we had enough participants to adequately test
our hypothesis.

With too few participants we may not be able to tell an important effect from
simple chance results. We will discuss this in the statistics section.
Foundations of
Research
Whom do you want to generalize to?
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Mammals
Humans
All Western people
Sampling a broader
population (i.e.,
larger sampling frame)
increases external
validity.
All Americans
Young Americans
College students
This College
This class
Sampling a more
specific or
smaller frame
generally
increases
internal
validity.
Foundations of
Research
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Who do you want to generalize to?
Samples typically represent targeted sub-populations


Demographic or ‘status’ groups;

Ethnicity, income or educational groups…

Geography; e.g., urban dwellers…



Medical / clinical groups; people with a
specific diagnosis or condition
Behavioral groups

Registered voters

Home owners

Ever used marijuana…
Targeting specific
groups increases
Internal validity by
decreasing the
complexity of the
sample.
…but may lessen
External validity by
narrowing the focus.
Groups defined by self-identification or subjective state

Views oneself as “highly likely to vote…”.

Above a ‘cut point’ on a stress or depression scale

“Conservatives” vs. “Liberals”
Foundations of
Research
Who is a group member?
Are you between 14 and 30 and have a
computer or smart phone available?
A = Yes
B = No
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Research
Who is a group member?
Do you use Facebook or other social
media 5 times a week or more?
A = Yes
B = No
C = Not sure – lost count.
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Research
Who is a group member?
Are you a “Facebook user”?
A = Yes
B = No
C = Not sure – let me
Facebook that.
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Research
Who is a group member?
Do you live in Pilson, Humboldt Park or other
neighborhood that is mostly Latino?
A = Yes
B = No
C = Maybe – I’m not sure
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Who is a group member?
Do you speak Spanish?
A = Yes
B = No
C =¿cuál era la pregunta?
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Foundations of
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Who is a group member?
Are you Latino?
A = Yes
B = No
C = Maybe – I’m not sure
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Foundations of
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Whom do you want to generalize to: who is in the group?

Once we choose our population group, we must
decide on criteria for membership…


To sample
sample social
social media
media users
users do I use a …
To

Rough demographic criterion?

Behavioral criterion (which behavior?)

Self-identification?
To sample “Latinos”…



Is geographic status specific enough?
Is Spanish language the defining
characteristic?
Can / must one call oneself “Latino” (even
if you do not speak Spanish…)?
Clearer and
narrower group
criteria increases
Internal validity
by making the
sample more
homogeneous.
Foundations of
Research
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Whom do you want to generalize to: who is in the group?
Once we choose our sampling group, we must
decide on criteria for membership…

To sample social media users do I use a …




Demographic criterion?
Some of these criteria are
easier to reliably measure
Behavioral criterion (which behavior?)
than others;
Self-identification?
To sample “Latinos”…



 Demographic variables
are often available in
census data
Is geographic status specific enough? Behavioral or subjective
Is Spanish language the defining
characteristic?
criteria require direct
assessment, and can be
less reliable.
Can / must one call oneself “Latino” (even if
you do not speak Spanish…)?
Foundations of
Research

Whom do you want to generalize to: who is in the group?
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Once we choose our sampling group, we must
decide on criteria for membership…

To sample social media users do I use a …




To



Rough demographic criterion?
 Of course different
criteria may yield very
Behavioral criterion (which behavior?)
different samples.
Self-identification?
 Our choice of sampling
criteria must be based on
sample “Latinos”…
our theory, hypothesis,
Is geographic status specific enough? or research question.
Is Spanish language the defining
characteristic?
Can / must one call oneself “Latino” (even if
you do not speak Spanish…)?
Foundations of
Research
Whom is in the group: Sampling criteria
Demographic or
‘status’ marker
Behavioral
criterion
Subjective / selfidentification
Who is a “Latino”?
Neighborhood
residence?
Spanish speaking?
Cultural practices?
Self-description?
# Hours registered
Describes occupation
as ‘student’
Who is a “Student”?
Lives on a campus
Who is “gay” or “lesbian”?
Lives same-sex 2person household?
Sexual or other
patterns?
Self-identification as
gay / lesbian?
Pattern of behaviors
and feelings?
Describes self as
“depressed”?
Who is “depressed”?
Received a diagnosis
from MH professional ?
Presents at Doctor’s
office for general
malaise?
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Sampling criteria
Demographic or
‘status’ marker
Behavioral
Subjective / selfidentification
Who is “Latino”?
Neighborhood
residence?
Spanish speaking?
Cultural practices?
Self-description?
# Hours registered
Describes occupation
as ‘student’
Who is a “Student”?
Lives on a campus
Who is “gay” or “lesbian”?
Lives same-sex 2person household?
Sexual or other
patterns?
Self-identification as
gay / lesbian?
Who is “depressed”?
 Each criteria may meet the goals of a particular
Received a diagnosis
Pattern
of behaviors
hypothesis
or empirical
question. Describes self as
from MH professional ?
and feelings?
“depressed”?
 Of course different
choices may lead to very
Presents at Doctor’s
different samples
office for general
 Some criteriamalaise?
are easy to assess but may be only
approximate
 Others may require relatively difficult assessments
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Foundations of
Research
Whom do you want to generalize to: Your “Sampling Frame”.

What is known about your larger population?

Are there Census or survey data?
 E.g., are there “population” data on depressed people?
 Do we know the demographic profiles of Facebook users?


Data about your target population will help you determine how
well your sample represents that population.
What is its size, sub-groups, location….
 Where / how can I best recruit members of the population?
 Will some sub-groups require different recruitment methods than
others?
 Will different recruitment methods be biased in favor of some subgroups?
 Internet surveys may be biased against older people.
 Studies that use monetary incentives pull for poorer people.
Foundations of
Research
Overview: From research question to sample
What is the research question?
 Are we describing some natural process?
 …testing a theory?
Target population
Sampling frame
What is the population of interest?
 What population does your research address?
 Whom do you want to generalize to?
The sample
Category of participant criterion?
 Demographic or “Status” criteria?
 Behavioral criterion?
 Self-Identification, attitudes or beliefs?
Operational definition of enrollment criteria?
 Inclusion criteria Requirements
Characteristicsfor
that
rule a participant
eligibility;
definition out,
of
e.g.,
mental
illness, previous
your severe
population
of interest.
 Exclusion Criteria:
exposure
to the experiment…
e.g.,
“young adult”,
20/20 vision, alcohol
Actual recruitment?
user…
 Concrete (operational) processes to recruit and enroll participants.
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Foundations of
Research
From theory to sample: Asthma
among African-Americans.
Study structure & research question:
EXAMPLE
 Adherence to a medication regimen is key to health among people
with asthma.
 Medication adherence is generally low, particularly among AfricanAmerican adolescents, who have high rates of asthma.
 Self-determination theory proposes that autonomous motivation
(being self-directed), self-confidence, and relatedness (family routines &
parental support) underlie adherence.
 This study tests the hypothesis that three variables comprising
self-determination theory will be associated with patients’
adherence to medications.
 Because young African-Americans have a significant health
burden from asthma, the study focuses on them.
Bruzzese, J., Idalski C, Lam, P, Deborah A.; Naar-King, S. (2014) Adherence to asthma medication regimens in
urban African American adolescents: Application of self-determination theory. Health Psychology, Vol. 33.5 (May
2014): 461-464. Article here.
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Foundations of
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From theory to sample: Asthma among African Americans.
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Population of interest?
 Young African-Americans who suffer from poorly controlled asthma.
EXAMPLE
Category of participant criterion?
 Demographic or Status criteria  Self-Identification / attitudes
not a criterion in this study.
 African-American adolescents
 Participating in long-term asthma control study.
 Behavioral criterion
 Poorly controlled asthma.
Operational definition of enrollment criteria?
 “Adolescent”: Age 10 – 18.
 “Poorly controlled”: At least one asthma-related hospitalization or two
asthma-related emergency department visits in the last 12 months.
Actual recruitment?
 N = 162 participants recruited from the hospital’s outpatient
immunology clinic after an asthma-related clinic visit or
hospitalization
Foundations of
Research
Results
 Having asthma regulation
EXAMPLE
embedded in the family
routine was the only predictor
of medication adherence.
Multiple regression analysis (all variables are tested simultaneously)
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Foundations of
Research
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Having a broad population helps with…
A = Avoiding confounds.
B = External validity.
C = Internal validity.
D = Specificity of the design.
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Research
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Having a more narrow population helps with…
A = Avoiding confounds.
B = External validity.
C = Internal validity.
D = Specificity of the design.
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It is not true that demographic, behavioral or
subjective sample criterion…
A = Must be based on the theory
or hypothesis you are testing.
B = Typically leads to the same
sample characteristics.
C = Requires different measures
to screen participants
D = Can substantially affect the
results of your study.
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Research
Click
What is a sampling frame?
A = Sample of the different stimuli that
will be used in the experiment.
B = The decision to use a behavioral
versus a self-identification or
subjective criteria for group
membership.
C = The list of sub-populations we
plan to study.
D = Census, survey or other data
about the target population that
allows us to know if our sample is
representative.
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Foundations of
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Research sampling
 Defining your target
population


Probability & Non-Probability
sampling methods.
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Major forms of sampling
Probability (Random) Sampling
 Recruit (or select) participants to maximize the representativeness of
the sample to a known population.
 Uses some form of random selection.
 Requires that each member of the population has a known (often
equal) probability of being selected.
 Most externally valid approach to sampling general populations
Non-Probability Sampling
 Use available samples for convenience, or targeted outreach to
unusual or small populations.
 Selection may be either systematic or haphazard, but is not random.
 Often the most externally valid approach to unusual, small, or extreme
groups, or groups where little is known.
 When used only for convenience it is the least externally valid.
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Watch that word ‘random’!
Participant
Selection
Sample
Random
Selection or a
Random Sample
refer to how we
recruit
participants; who
is in the sample.
Participant
Assignment
Experimental
Experimental
Treatment or
Procedures Manipulation
Results
Group A 
Procedure 
Treatment 
Outcome
Group B 
Procedure 
Control 
Outcome
(Group C) 
(Procedure ) 
(Alternate 
Treatment?)
(Outcome)
Random
Assignment is
how we (should)
assign
participants to
different groups.
Foundations of
Research
Probability / Random Sampling
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• Core feature: all members of the study population have an equal (or
known) chance of being sampled
• Procedure: Choose participants in a systematic, random fashion.
• e.g., every 100th student ID,
• Every 1000th person on a voter registration record.
• Random digit dialing for telephone surveys.
• Advantages: eliminates obvious biases of convenience sampling
• Limitations:
• May under-sample unusual / hard to reach participants
• Some may be unavailable in, e.g., telephone lists, computer
files.
Foundations of
Research
Basic Forms of random sampling
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• Simple Random Sampling: All members of the population have
about equal chance of selection.
• Multi-Stage: Randomly select population units (census tracts,
households, schools..), then randomly select individuals within unit.
• Stratified or Cluster: Random selection within:
• Population blocks;
 E.g., gender (randomly select 50 women and 50 men),
 Ethnicity, (…25 African-Americans, Caucasians, Latinos, Asians, etc.)
• Venues or events;
 E.g., randomly select 40 men & women at Lollapalooza…
Foundations of
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Simple Random sampling
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Objective: Attempts to truly represent the general population; absolute
minimal selection bias.
Procedure: Recruitment method where all members of the population
have ≈ chance of being selected:

Examples:

Gallup polls or surveys using random
digit dialing
“Long form” of the census to a small %
of U.S. households
Advantages:
Most representative sampling frame for the general population
Disadvantages:
Any recruitment method excludes some people (no telephone,
no stable address, etc.).
Very expensive for face-to-face (non-telephone or internet)
data collection.
Foundations of
Research
Multi-stage random sampling
Objectives:
Develop a focused & efficient random sample.
Use random sampling “stages” to reach
hidden, stigmatized or other hard to reach
groups.
 Simple random sampling is optimal.
 However, it is biased when relying on telephone, internet or
similar contact methods.
 A simple random sample for face-to-face interviews or
recruitment for a larger research study (e.g., a public health
intervention) is prohibitively expensive.
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Multi-stage random sampling
Selecting a random sample of
college students:
Rather than trying to randomly
sample the entire
population of students, we
narrow our approach.
 Randomly select subpopulation blocks.
General population
Universities
Classes
 Randomly select smaller
groups within them…
 Randomly select students
within the target classes...
 To arrive at a efficient
random sample.
Students
Final sample
Foundations of
Research
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Multi-stage random sampling
“Real World” example: NIDA*
household surveys of drug
use.
 Randomly select a
moderate # of census
tracts.
 Randomly select modest #
of households within each
target tract…
 Interview the first adult who
answers the phone in each
household...
 To arrive at a efficient
random sample.
*National Institute on Drug Abuse
General population
Step 1.
Step 2.
Step 3
Final sample
Foundations of
Research
Stratified or cluster sampling
Objective: Represent every key segment of the population.
Procedure:
 Decide which population segments are important;
 E.g., ethnic groups,
 Geographic areas,
 Self-identification.
This decision is based on your hypothesis
or empirical question.
 Randomly select from each segment.
 Proportionate: Sampling fraction from each segment should
approximate the overall population.


This is the distribution of ethnicity
in the U.S.
A stratified sample would randomly
select from each ethnic group to
approximate this distribution.
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Research
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Stratified or cluster sampling
Objective: Represent every key segment of the population.
Procedure:
Decide which population segments are important;
Randomly select from each segment.
 Proportionate: Sampling fraction from each segment should
approximate the overall population.
 Dis-Proportionate: Over-sampling population groups to ensure
you have large enough samples of small groups.
This estimates the distribution
of legal vs. illegal immigrants.
To directly compare groups we
over-sample illegal immigrants.
Foundations of
Research
Probability Sampling overview
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Core features:

Random selection of participants from the population

Most externally valid approach.

Assumes:  A clear sampling frame.
Summary
 All segments of the population are available.
Variations:
Each member of the entire population has an ≈ chance of being
selected.
 Simple
b.
Select segments of the population, e.g., census tracts,
registered voters.
Each segment member has an ≈ chance of being selected.
 Cluster
a.
b.
Identify clusters, e.g. sports fans at a sports bar.
Each cluster member has an ≈ chance of being selected.
 Stratified
a.
b.
Identify strata, e.g., ethnic groups, gender, age groups..
Randomly select a proportion of each strata.
 Multi-stage
a.
Foundations of
Research
Non-Probability Sampling
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Useful for populations that:
Cannot be randomly sampled; “hidden” or difficult to reach.
No sampling frame available, such as census data, describing its
size, composition, etc.
 Examples: drug users, recent immigrants, gay men…
Likely to misrepresent the population
 It may be difficult or impossible to detect this misrepresentation.
 Can be over-sensitive to incentives: paying participants attracts more
poor people.
 “Respondent Driven” sampling (RDS) allows for “targeted” population
estimates.
Foundations of
Research
Types of Non-probability Samples

Haphazard

Modal instance

Venue – time / space

Multi-frame

Snowball / Respondent driven

Web

Heterogeneity
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Foundations of
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Haphazard Sampling
“Man on the street”; recruiting the most easily available
participants.



Literally recruiting in public places for, e.g., brief interview

College psychology majors.

Medical / therapy clients in a clinic.
Often used for quick interview / attitude studies on
current topics.
Advantage: Participants are readily available.
Problem:
No evidence for representativeness.
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Modal Instance Sampling
Recruit a “typical case” of a target population.


A member of a target population group;

…a ‘typical’ college student with debt.

These methods often used
An injection drug user, homeowner, etc.
A person affected by a major event;

A 9/11 survivor...

A witness to a natural disaster.
by journalists.
 New York Times: “down low”
sexual patterns in AfricanAmerican men.
 Often used to describe an event or way of life, or to
generate hypotheses for later research.
Advantage: Direct, personal description.
Problems:
Potentially strong self-selection bias in who volunteers
for such personal disclosure
Social desirability responding or biased recall may
compromise accuracy of answers.
Foundations of
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Targeted Multi-frame Sampling

Sample a specific, hard to reach group…

..no census or similar data for sampling frame.


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
No clear “population blocks” to use in a multi-stage sample,

Population spread among many venues or locations…

... more or less sensitive to different recruitment approaches.
Multiple (convenience) sampling “frames”:

Direct outreach.

Newsletters, internet lists, chat rooms

Organizations or meeting places.
Most common & valid convenience sample
Foundations of
Research
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Targeted Multi-frame Sampling

Sample a specific, hard to reach group

No census or similar data for sampling frame.

Uses multiple (convenience) sampling “frames”.
Example: recruiting gay men for HIV research
We use multiple ways to approach men or for them to contact us.
Multi-frame sample

Direct outreach in
bars, clubs, street.

Community events
(festivals…)

Newspaper ads.



Flyers in bars &
stores.
Medical clinics.
“Snowball” / word of
mouth.
Foundations of
Research
Targeted Multi-frame Sampling

Sample a specific, hard to reach group

No census or similar data for sampling frame.

Uses multiple (convenience) sampling “frames”.
 Different people are available through different “frames”.
 This approach recruits a broad cross-section of the population.
 One of the more common forms of convenience sampling.
 It also allows us to “test” different
sample venues.
 In my HIV research:
 Riskier men came from bar or
street outreach;
 Minorities responded more to personal
contact such as snowball recruitment.
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Foundations of
Research
Snowball / Network sampling.

An initial set of participants are paid to recruit others, who
are paid to recruit still others, etc.

Targeted sampling: participants use eligibility criteria to
recruit others.

Recruit network of “linked” people;

A network of, e.g., drug users, gamblers, sexual partnerships…

Population of an organization or school…
Problem: Sensitive to incentives!
Advantage: Access unusual or “hidden” people related by a
common behavior or venue.

Using participants to recruit others they know may yield a
more representative sample.

Researchers can collect data on who recruits whom to
examine the structure of social networks
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Foundations of
Research
Example of social network sampling:
Bearman et al., Romantic ties among adolescents
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With a number of
smaller chains
And a small % in 2 to
4 person chains


A substantial majority of students are
in an extended, linked chain of
relationships.
From sampling
perspective,
several “seeds”
access most of
the population
Findings suggest
a clear potential
for STI
transmission.
Foundations of
Research
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Web Sampling.
Typically highly targeted samples:
 Gay / bisexual men…;
 Adolescents…;
 “Gamers”…
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Typically access through existing venues:
 Users of specific web sites;
 List-serves, e-mail lists;
 Active recruitment in “chat rooms”.
Problem:
Inherent bias in computer literacy.
Advantage:
 Cheap large national sample;
 Access unusual or “hidden” people who reach
others via internet.
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Foundations of
Research
Heterogeneity Sampling.
• Sample every sector of a population -- at least several of
everyone -- without worrying about proportions.
• At least some members of each geographic area
• …ethnic group
• …behavioral group (voters & non-voters…)
• Assume that a few people are a good proxy for the group.
Examples: focus groups or qualitative interviews about products,
social issues...
Problem; Cannot be sure a few people really represent their
sub-group.
Advantage: At least some representation of all sub-groups.
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Foundations of
Research
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A probability sample is…
A = Based on some form of random selection.
B = Always representative of the population
C = Best for any population
D = Is usually easier to collect than other sample approaches.
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Foundations of
Research
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A Gallup poll or telephone survey is a…
A = Simple random sample.
B = Multi-stage random sample.
C = Social network or “snowball” sample.
D = Haphazard sample.
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Foundations of
Research
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Respondent-driven sampling, where target
people recruit people like them, is a…
A = Simple random sample.
B = Multi-stage random sample.
C = Social network or “snowball” sample.
D = Haphazard sample.
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Foundations of
Research
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My distributing a survey to this class is a…
A = Simple random sample.
B = Multi-stage random sample.
C = Social network or “snowball” sample.
D = Haphazard sample.
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Foundations of
Research
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Selecting every 100th registered voter and
contacting them for a survey is a…
A = Simple random sample.
B = Multi-stage random sample.
C = Social network or “snowball” sample.
D = Haphazard sample.
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Foundations of
Research
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Randomly selecting classes across the
university, than sampling each 3rd person, is a…
A = Simple random sample.
B = Multi-stage random sample.
C = Social network or “snowball” sample.
D = Haphazard sample.
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Foundations of
Research
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A non-probability sample…
A = Is perfectly OK if you have limited resources.
B = Just consists of grabbing the most convenient possible
participants.
C = Is never adequate to generalize from.
D = Can be best for hard to reach or unusual participants.
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Foundations of
Research
Sampling overview
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Who do you want to generalize to?
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Summary
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Who is the target population?
 broad – external validity
 narrow – internal validity
How do you decide who is a member?
 demographic / behavioral criteria?
 subjective / attitudinal?
What do you know about the population already – what
is the “sampling frame”.
Is a Probability or random sample possible?
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“Hidden” population?
Socially undesirable research topic?
Easily available via telephone, door-to-door?
Sampling frame adequate to choose selection method?
Foundations of
Research
Overview, 2
Summary
Types of Non-probability Samples:

Haphazard,
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Modal instance,
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Venue – time / space,
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Multi-frame,
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Snowball / Respondent driven,
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Web,
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Quota,
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Heterogeneity.
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Foundations of
Research
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Overview, 3
Probability sampling
 Most externally valid
Summary
 Assumes:
 Clear sampling frame
 Population is available
 Less externally valid for
hidden groups.
Non-probability sampling
 Less externally valid
 High “convenience”
 Best when:
 No clear sampling frame
 Hidden / avoidant
population.