Educational Research:
Fundamentals for the Consumer
Sixth Edition
Woolfolk / Perry
Child and Adolescent Development
© 2012 Pearson Education, Inc. All rights reserved.
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
Participants and Sampling
Chapter 4
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
Discussion Topics

Participants and sampling

Sampling for quantitative studies

Sampling for qualitative studies

Sampling for mixed-method studies

Issues related to sampling

Criteria for evaluating sampling
procedures
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
3
Participants and Samples

Participant
• Person from whom data are collected



Can be thought of as source of evidence or unit
of study
Also referred to as a subject
Sample – the collective group of
participants from whom data are
collected
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
4
Quantitative Sampling
Procedures

Two types of procedures
• Probability


Statistically driven sampling techniques where
the probability of being selected is known
Purpose is to select a group of participants
representative of the larger group of subjects
from which they are selected
• Non-probability

Pragmatically driven sampling techniques where
the probability of being selected is not known
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
5
Probability Sampling

Method of sampling in which participants are
selected randomly from a population in such a
way that the researcher knows the probability
of selecting each participant.
• In a sample of 10 from a population of 100, each
subject has a 10% chance of being included in the
sample
• In a sample of 50 from a population of 100, each
participant has a 50% chance of being in included in
the sample
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
6
Probability Sampling

Population: a large group of individuals to whom the
results of a study can be generalized
• Target population: the group to whom the results are
intended to be generalized
• Sampling frame (i.e., survey population)


The group to whom the researcher has access and from
which the actual sample will be drawn
Often the sampling frame and the target population are
different
• Margin of error
 The interval (typically 95% probability) within which the true
population lies
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
7
Probability Sampling

Types of probability techniques
• Simple random - a number is assigned to
each subject in the population and a table
of random numbers or a computer is used
to select subjects randomly from the
population
• Systematic - a number is assigned to each
subject in the population, and every nth
member of the population is selected
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
8
Probability Sampling

Types of probability techniques
• Stratified sampling - similar to random sampling
with the exception that subjects are selected
randomly from strata, or subgroups, of the
population

Strata: homogeneous subgroups within a population
Males and females
– Certified and non-certified teachers
–


Proportional stratified sample
Disproportional stratified sampling
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
9
Probability Sampling

Types of probability techniques
• Cluster sampling: similar to random
sampling except that naturally occurring
groups are randomly selected first, then
subjects are randomly selected from these
sampled groups


Useful when it is impossible to identify all of the
individuals in a population
Typical educational clusters are districts, schools,
or classrooms
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
10
Non-Probability Sampling

Method of sampling in which the probability of
selecting a participant is unknown
• It is often not possible to use probability sampling
techniques due to access, time, resource or financial
constraints
• Three categories of non-probability sampling
procedures



Convenience sampling
Quota
Purposive (discussed in qualitative section)
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
11
Non-Probability Sampling

Convenience sampling: selecting a participant
or group of participants based on their
availability to the researcher
• Typical of much educational research given the
constraints under which it is conducted
• The major concern is the limited generalizability of
the results from the sample to any population
• Examples
 Students enrolled in the researcher’s classes
 Fourth-grade students in two local, parochial schools
to which the researcher has access
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
12
Non-Probability Sampling

Quota sampling: non-random sampling
representative of a larger population
• Used when the researcher cannot use
probability sampling procedures but does
want a sample that is somewhat
representative of the population
• Similar to stratified sampling with the
exception that the subjects are selected
non-randomly
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
13
Qualitative Sampling
Procedures

Purposive sampling: selection of particularly
informative or useful participants
• Typically selects information-rich participants who
are studied in-depth
• Also known as purposeful sampling
• Examples
 It is reasonable to select “expert” teachers if one is
trying to understand how teachers use effective
instructional strategies
 It is reasonable to select physically fit individuals if
one is trying to identify effective exercise behaviors
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
14
Purposeful Sampling

Types of purposeful techniques
• Criterion: selecting individuals with certain
•
•
•
•
•
important characteristics
Typical case: selecting a representative participant
Extreme case: selecting a unique or atypical
participant
Maximum variation: selecting participants to
represent extreme cases
Snowball (i.e., network): selecting participants from
recommendations of other participants
Critical case: selecting the most important
participants to understand the phenomena being
studied
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
15
Mixed-method Sampling
Procedures

Stratified Purposeful Sampling
•
•

Purposeful Random Sampling
•

Random sampling first, then the cases become purposeful
Concurrent Sampling
•

Stratified sampling first, then a few cases examined intensely
Purposeful sampling first, then stratify on a variable
Quantitative and qualitative portions are done independently
Multilevel
•
Sample is representative of different levels, e.g., classrooms,
schools, and districts.
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
16
Using Sampling Procedures

Quantitative studies
• The desired use of probability sampling due to the
ability to generalize the results to the larger
population
• Frequent use of non-probability techniques particularly convenience sampling - due to access,
time, resource, or financial constraints

Qualitative studies
• Almost exclusive reliance on purposeful techniques

Mixed-method studies
• Relies on both and hybrid approaches of quantitative
and qualitative procedures
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
17
Sampling and Results

How might the sampling procedures affect the
results?
• Need to identify the sampling procedure used
• Need to evaluate the sampling procedure in light of
the research problems and conclusions
• Need to consider the strengths and weaknesses of
specific sampling procedures (see Table 4.2)
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
18
Sampling and Results

How might the characteristics of the
participants affect the results?
• Volunteer samples
 Different characteristics between volunteers and nonvolunteers can lead to different responses
Educational level
– Socio-economic status
– Need for social approval
– Conformity
–

Commonly used due to their availability
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
19
Sampling and Results

Sample size - general rules of thumb
• Quantitative studies
 30 participants for correlational research
 15 participants in each group for experimental research
• Qualitative studies - a sufficient number of
participants are needed to ensure that no new
information is forthcoming from additional cases
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
20
Sampling and Results

Sample Size
• Need to interpret results very carefully -
results form studies using very large or very
small samples can be misleading


Results indicating “no difference” or “no
relationship” in studies with small samples an be
problematic
Results of “differences” or “relationships” in
studies can be problematic
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
21
Sampling and Results

Subject motivation
• Specific characteristics of the sample can
predispose them to respond in certain ways


Only selecting teachers using holistic language
strategies would likely predispose them to
respond favorably to an attitudinal scale focusing
on holistic language instruction
Only selecting students who participate in extracurricular activities might predispose them to
certain types of responses
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
22
Sampling and Results

Sampling bias: the difference between the
“observed” and “true” results that is attributed
to the sampling mistakes of the researcher
• Deliberately sampling participants with certain
attributes



Positive attitudes
High self-esteem
High level of achievement
• Using participants from different populations and
assigning them to different treatment groups

Males to an experimental treatment group and females
to a traditional treatment group
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
23
Criteria for Evaluating
Sampling Procedures





Participants should be described clearly with
specific and detailed information related to
demographic and other personal characteristics
The population should be clearly defined.
The sampling procedure should be clearly
described.
Response rate for survey studies should be
indicated.
Selection of participants should be free of bias.
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
24
Criteria for Evaluating
Sampling Procedures




Selection procedures should be appropriate for
the problem being investigated.
Adequate sample sizes should be used.
Qualitative studies should have informative
and knowledgeable subjects.
Mixed-method studies should clearly indicate
samples used for each phase of the study
McMillan
Educational Research: Fundamentals for the Consumer, 6e
© 2012 Pearson Education, Inc. All rights reserved.
25