Foundations of
Research
1
Psychology 242, Dr. McKirnan
Research Sampling.
 Defining your target
population
 Probability & NonProbability sampling
methods.
Dr. David J. McKirnan, University of Illinois at
Chicago, Psychology; [email protected]
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Foundations of
Research
The big picture: Research sampling
 Define your target population

 What group do you want to generalize to?
 How is / is not a member of the group?
 What is your sampling frame?
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Foundations of
Research
3
Sampling
Sampling: Who do you want to generalize to?

Any study assesses only a sample of the
population.

There are many different ways we may collect a
sample.

There are many different populations or subpopulations we may be interested in.

The size and breadth of a sample can affect the
Internal or External validity of the study.
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research
4
Define the target population
Who do you want to generalize to?
Mammals
Humans
All Western people
Breadth of
population to sample
from (i.e., size of
sampling frame).
Represents
increasing external
validity.
Psychology 242, Dr. McKirnan
All Americans
Young Americans
College students
UIC Students
This class
Week 6; Sampling
Specificity (and
ease) of sampling
frame.
Generally
increases
internal
validity.
Foundations of
Research
5
Who do you want to generalize to?
Samples typically represent targeted sub-populations




Demographic or ‘status’ groups;

Ethnic or Socio-economic status groups

Geography; e.g., urban dwellers…

Medical / clinical groups; specific diagnosis
Behavioral groups

Registered voters

Home owners

Ever used marijuana…
Targeting specific
groups increases
Internal validity by
decreasing the
complexity of the
sample.
Groups defined by self-identification or
subjective state

“Conservatives” vs. “Liberals”

Above a ‘cut point’ on a stress or depression scale

Views oneself as “highly likely to vote…”.
Psychology 242, Dr. McKirnan
Foundations of
Research
Research samples & validity
EXAMPLE
Clinical drug trials illustrate the
conflict between internal v.
external validity in sampling.

People with diverse symptoms and
backgrounds see physicians for
depression.

To enhance internal validity drug
researchers use exclusion criteria to
select only participants who fit a
specific definition of depression

Zimmerman et al. suggest that too
many exclusion criteria compromises
the validity of this research area. (click
image for article)
Zimmerman, M.l, Mattia, J.I., & Posternak, M.A. (2002). Are Subjects in Pharm-acological Treatment
Trials of Depression Representative of Patients in Routine Clinical Practice? Am J Psychiatry, 159,
469–473.
Week 6; Sampling
Psychology 242, Dr. McKirnan
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Foundations of
Research
Exclusion criteria & validity
EXAMPLE
The study begins with a large # of
people self-referred for depression
They exclude those with serious
mental illness, drug abuse or
personality disorder…
…whose symptoms are not severe
enough, are suicidal, or who have
other affective disorders..
…whose symptoms are too recent OR
too long-standing…
…and end up with a small, carefully
selected sub-set of patients (8.4% of
general depression patients).
Psychology 242, Dr. McKirnan
Week 6; Sampling
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EXAMPLE
Foundations of
Research
External vs. internal validity in sampling

Applying rigorous study selection
criteria for drug trials excludes the
great majority of routine depression
patients.

Rigorous participant selection for
internal validity seriously
compromises external validity in
these studies.

This leaves the actual usefulness of
anti-depressant (and other)
medications for the general
population in doubt.

To be useful research must balance
the need for careful subject selection
with the need for representativeness
Psychology 242, Dr. McKirnan
Week 6; Sampling
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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
C = Not sure – lost count.
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
Who is a group member?
Do you use Facebook or other media 5
times a week or more?
A = Yes
B = No
C = Not sure – lost count.
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
Who is a group member?
Are you a “Facebook user”?
A = Yes
B = No
C = Not sure – let me
Facebook that.
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
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
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
Who is a group member?
Do you speak Spanish?
A = Yes
B = No
C =¿cuál era la pregunta?
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
Who is a group member?
Are you Latino?
A = Yes
B = No
C = Maybe – I’m not sure
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
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Define the target population
Who 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 …

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
16
Define the target population
Who 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 …




Rough 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
Define the target population
17
Who 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 …




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
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Sampling criteria
Demographic or
‘status’ marker
Behavioral
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?
Dr. David J McKirnan
Sampling
Foundations of
Research
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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
Dr. David J McKirnan
Sampling
Foundations of
Research
Overview: From research question to sample
What is the research question?
 Are we describing some natural process?
 …testing a theory?
What is the population of interest?
 What population or subpopulation is relevant to our research
question?
 Whom do we want to generalize to?
Category of participant criterion?
 Demographic or “Status” criteria?
 Behavioral criterion?
 Self-Identification, attitudes or beliefs?
Operational definition of enrollment criteria?
 Specific measures
Actual recruitment?
 Concrete processes to recruit and enroll participants.
Psychology 242, Dr. McKirnan
Week 6; Sampling
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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.
Psychology 242, Dr. McKirnan
Week 6; Sampling
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Foundations of
Research
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
 African-American adolescents  Self-Identification / attitudes
 Poorly controlled asthma.
not a criterion in this study.
 Behavioral criterion
 Already participating in long-term asthma control study.
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?
 Recruited from the hospital’s outpatient immunology clinic after an
asthma-related clinic visit or hospitalization
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research
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Results
 Having asthma regulation embedded in the family
EXAMPLE
routine was the only predictor of medication adherence.
Multiple regression analysis (all variables are tested simultaneously)
Dr. David J McKirnan
Sampling
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Foundations of
Research
Who 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?
 E.g., internet surveys are biased against less computer-oriented
people.
Foundations of
Research
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Click
Having a broad population helps with…
A = Avoiding confounds.
B = External validity.
C = Internal validity.
D = Specificity of the design.
Dr. David J McKirnan
Sampling
Foundations of
Research
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Click
Having a more narrow population helps with…
A = Avoiding confounds.
B = External validity.
C = Internal validity.
D = Specificity of the design.
Dr. David J McKirnan
Sampling
Foundations of
Research
27
Click
It is not true that using a behavioral or objective
versus subjective or ψ 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.
Dr. David J McKirnan
Sampling
Foundations of
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 selfidentification 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.
Dr. David J McKirnan
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Foundations of
Research
Research sampling
 Defining your target
population


Probability & NonProbability sampling
methods.
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
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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Foundations of
Research
Participant
Selection
Sample
Random
Selection or a
Random Sample
refer to how we
recruit
participants; who
is in the sample.
Psychology 242, Dr. McKirnan
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Watch that word ‘random’!
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.
• 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.
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research
Basic Forms of random sampling
• Simple Random Sampling: Select a specific % of a target
population; all members of population have about equal chance
of selection.
• Multi-Stage: Randomly select population units (census tracts,
households, schools..), then randomly select individuals within unit.
• Stratified: Random within population sub-blocks, e.g., gender
(randomly select 50 women and randomly select 50 men), ethnicity, etc.
• Cluster: Random within (potentially convenience) clusters, e.g.,
specific locations or “venues”, events, times of day, etc.
Psychology 242, Dr. McKirnan
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Foundations of
Research
Simple Random sampling
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 using random digit dialing
surveys
“Long form” of the census to a small %
of U.S. households
Advantages: Most representative sampling frame for general
(non-targeted) population
Disadvantages: Any recruitment method excludes some people
(no telephone, no stable address, etc.).
Psychology 242, Dr. McKirnan
Week 6; Sampling
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Foundations of
Research
Multi-Stage Random sampling
35
Objective: Focused & efficient random sample.
Procedure: Concentrate recruitment in specific locations or venues.
Examples:
NIDA household drug surveys:
1) Random select moderate # of
census tracts nationally
2) randomly select small % of
households within each tract;
3) Interview 1st adult who answers
phone in each household
“CITY” HIV study among youth:
1) Randomly select bars, clubs,
other venues across the city
2) Randomly select days & times to
recruit in them.
3) Randomly approach every 4th
person who enters the venue for
an interview
Advantage: Much more efficient that simple random
Disadvantage: Same as above; necessarily excludes some people
Bias in who answers phone in drug-using households…?
Not all young gay men go to bars or similar venus…
Psychology 242, Dr. McKirnan
Week 6; Sampling
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 must be based on
your hypothesis or empirical
question.
 Randomly select from each segment.
 Proportionate: Same sampling fraction from each segment;
approximates overall population
• e.g., sample 1% of all African-Americans, 1% of all Latinos…
 Disproportionate: Unequal sampling fraction across segments, to
over-represent smaller groups
• e.g., select larger % of recent immigrants…
Psychology 242, Dr. McKirnan
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Foundations of
Research
Non-Probability Sampling
37
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
 May be difficult or impossible to detect this misrepresentation
 Often over-sensitive to incentives: paying participants
attracts more poor people
 “Respondent Driven” sampling (RDS) allows for “targeted” population
estimates
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research

Non-Probability methods (1)
Haphazard Sampling; “Man on the street”

College psychology majors

Available medical / therapy clients

Volunteer samples
Problem: No evidence for representativeness
Advantage: availability of participants
Modal Instance Sampling; “Typical” case
 Typical New Yorker describing trade tower tragedy
 Typical voter.
Problem: May not represent the modal group.
Advantage: Describe simple, “typical case”
Haphazard / Modal instance often used by journalists or
qualitative-descriptive studies; see NYT “down low” article.
Psychology 242, Dr. McKirnan
Week 6; Sampling
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Foundations of
Research
Non-Probability methods, 2
Venue & time / space Sampling

Sample a specific, well-defined, often hard to reach group

Assume group members are well represented at specific
locations or settings (“venues”).


Use “Intercept” methods for reaching participants

Use indigenous outreach workers from the population

Develop a standard recruitment script

Collect / distribute contact information for later participation
Time / Space randomization:


Lessen bias due to choice of venue:

Randomly approach different venues at different times

Randomly select participants within the venue (e.g., every 4th person…)
Strategy must be based on a clear epidemiological or theory
question.
Examples: Shopping mall intercepts, gay recruitment
Psychology 242, Dr. McKirnan
Week 6; Sampling
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Foundations of
Research
Outreach / venue sampling: examples of palm cards
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
Outreach lead sheet
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
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Foundations of
Research
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Non-Probability methods, 3
Targeted Multi-Frame Sampling

Sample a specific, hard to reach group

No census or similar data for sampling frame.

Uses multiple (convenience) sampling “frames”:

Direct outreach to places where population members are available
(venue sampling).

Newsletters, internet lists & chat rooms

Organizations or meeting places

Strategy must be based on a clear epidemiological or
theory question.

Most common & valid convenience sample
Examples:


“MTV” Market segments
Shoplifters
Psychology 242, Dr. McKirnan
Week 6; Sampling


People who have risky sex
Homeless people…
Foundations of
Research
43
Non-Probability methods (3)
Snowball / “Respondent Driven” Sampling (RDS)

Early participants are paid to recruit others, who
recruit others, etc.
Choice of seeds.

Form of targeted sampling:

Recruit network of “linked” people tracked by referrals
Problem:
Eligibility criteria
Sensitive to incentives!
Advantage: Access unusual or “hidden” people related
by a common behavior.




With enough “generations” of links can well represent a
target population.
Often part of multi-frame approach.

With RDS can show “chain” of referrals / links.
Useful for people who mistrust research or where personal
contact is necessary for recruitment (HIV, drug use).
Portrays “chain” of influence or, e.g., infectious disease.
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research
RDS coupon examples
Heckathorn, D.D. & Magnani, R. (2004). Snowball and RespondentDriven Sampling. In: Behavioral Surveillance Surveys: Guidelines for
Repeated Behavioral Surveys in Populations at Risk of HIV
Psychology 242, Dr. McKirnan
Week 6; Sampling
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Foundations of
Research
RDS; chain description
Heckathorn, D.D. & Magnani, R. (2004). Snowball and Respondent-Driven Sampling. In:
Behavioral Surveillance Surveys: Guidelines for Repeated Behavioral Surveys in Populations at
Week 6; Sampling
Psychology
242, Dr. McKirnan
Risk of HIV.
45
Foundations of
Research
Example of social network sampling:
Bearman et al., Romantic ties among adolescents
46
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.
Psychology 242, Dr. McKirnan
Week 6; Sampling
From sampling
perspective,
several “seeds”
access most of
the population
Findings
suggest a clear
potential for STI
transmission.
Foundations of
Research

Non-Probability methods
Quota Sampling
Similar to cluster sampling, except
you cannot randomly sample each
Select people non-randomly
according to quotas
population segment.
 Must have clear theory / research question to pick
relevant population characteristic(s).
 Proportional quota sampling
• Represent major characteristics of a population. If gender is
important, and the proportion of women :: men in your
population = 65% :: 35%, the sample must meet that quota.
 Non-proportional quota sampling
• Sample enough members of each group to test hypothesis,
even if the sample is not proportional. (e.g., recruit 50 women &
50 men, even though the real proportion is 65::35).
• Helps assure that you have good representation of smaller
population groups.
Psychology 242, Dr. McKirnan
Week 6; Sampling
47
Foundations of
Research

Non-Probability methods
Web sampling
Typically highly targeted samples
 Gay / bisexual men…
 Adolescents…
 “Gamers”…

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
Psychology 242, Dr. McKirnan
Week 6; Sampling
48
Foundations of
Research
Non-Probability methods;
Heterogeneity Sampling
49
• 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 subgroups.
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research
50
Click
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.
Dr. David J McKirnan
Sampling
Foundations of
Research
51
Click
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.
Dr. David J McKirnan
Sampling
Foundations of
Research
52
Click
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.
Dr. David J McKirnan
Sampling
Foundations of
Research
53
Click
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.
Dr. David J McKirnan
Sampling
Foundations of
Research
54
Click
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.
Dr. David J McKirnan
Sampling
Foundations of
Research
55
Click
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.
Dr. David J McKirnan
Sampling
Foundations of
Research
56
Click
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.
Dr. David J McKirnan
Sampling
Sampling overview
Foundations of
Research
57
Who do you want to generalize to?

Summary


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?




“Hidden” population?
Socially undesirable research topic?
Easily available via telephone, door-to-door?
Sampling frame adequate to choose selection method?
Psychology 242, Dr. McKirnan
Week 6; Sampling
Foundations of
Research
58
Overview, 2
Summary
Types of Non-probability Samples








Haphazard
Modal instance
Venue – time / space
Multi-frame
Snowball / Respondent driven
Web
Quota
Heterogeneity
Psychology 242, Dr. McKirnan
Week 3; Experimental designs
Foundations of
Research
Probability sampling
 simple
 multi-stage
 cluster or stratified
Summary
59
Overview, 3
Non-probability sampling
 targeted / multi-frame
 snowball
 quota, etc.
 Most externally valid
 Assumes:
 Clear sampling frame
 Population is available
 Less externally valid for
hidden groups.
 Less externally valid
 High “convenience”
 Best when:
 No clear sampling frame
 Hidden / avoidant
population.
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