Sampling Techniques
& Samples Types
Outlines
Sample definition
Purpose of sampling
Stages in the selection of a sample
Types of sampling in quantitative researches
Types of sampling in qualitative researches
Ethical Considerations in Data Collection
The process of selecting a number of individuals
for a study in such a way that the individuals
represent the larger group from which they were
selected
STUDY POPULATION
SAMPLE
TARGET POPULATION
4
A sample is a smaller (but hopefully
representative) collection of units from a
population used to determine truths about that
population (Field, 2005)
The sampling frame
A list of all elements or other units containing the
elements in a population.
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Population
the larger group from which
individuals are selected to
participate in a study
Target population
A set of elements larger than or different
from the population sampled and to which
the researcher would
like to generalize
study findings.
To gather data about the population in order
to make an inference that can be generalized
to the population
Stages in the
Define the target population
Selection
of a Sample
Select a sampling frame
Determine if a probability or nonprobability
sampling method will be chosen
Plan procedure for selecting
sampling units
Determine sample size
Select actual sampling units
Conduct fieldwork
Purpose to identify participants from
whom to seek some information
Issues
Nature of the sample (random samples)
Size of the sample
Method of selecting the sample
Important issues
Representation the extent to which the
sample is representative of the population
Generalization the extent to which the
results of the study can be reasonably
extended from the sample to the population
Sampling error
The chance occurrence that a randomly selected
sample is not representative of the population
due to errors inherent in the sampling technique
Important issues (continued)
Sampling bias
Some aspect of the researchers sampling
design creates bias in the data.
Three fundamental steps
Identify a population
Define the sample size
Select the sample
Non-
Probability
samples probability
samples
Known as probability sampling
Best method to achieve a representative
sample
Four techniques
1. Random
2. Stratified random
3. Cluster
4. Systematic
1. Random sampling
Selecting subjects so that all members of a population have an
equal and independent chance of being selected
Advantages
1. Easy to conduct
2. High probability of achieving a representative sample
3. Meets assumptions of many statistical procedures
Disadvantages
1. Identification of all members of the population can be
difficult
2. Contacting all members of the sample can be difficult
Random sampling (continued)
Selection process
Identify and define the population
Determine the desired sample size
List all members of the population
Assign all members on the list a consecutive number
Select an arbitrary starting point from a table of
random numbers and read the appropriate number of
digits
2. Stratified random sampling
The population is divided into two or
more groups called strata, according to
some criterion, such as geographic
location, grade level, age, or income,
and subsamples are randomly selected
from each strata.
Stratified random sampling (continued)
Advantages
More accurate sample
Can be used for both proportional and non-
proportional samples
Representation of subgroups in the sample
Disadvantages
Identification of all members of the population can
be difficult
Identifying members of all subgroups can be
difficult
Stratified random sampling (continued)
Selection process
Identify and define the population
Determine the desired sample size
Identify the variable and subgroups (i.e., strata) for
which you want to guarantee appropriate
representation
Classify all members of the population as members
of one of the identified subgroups
3. Cluster sampling
The process of randomly selecting intact groups, not
individuals, within the defined population sharing similar
characteristics
Clusters are locations within which an intact group of
members of the population can be found
Examples
Neighborhoods
School districts
Schools
Classrooms
Cluster sampling (continued)
Advantages
Very useful when populations are large and spread over a
large geographic region
Convenient and expedient
Do not need the names of everyone in the population
Disadvantages
Representation is likely to become an issue
Cluster sampling (continued)
Selection process
Identify and define the population
Determine the desired sample size
Identify and define a logical cluster
List all clusters that make up the population of
clusters
Estimate the average number of population members
per cluster
Determine the number of clusters needed by dividing
the sample size by the estimated size of a cluster
Randomly select the needed numbers of clusters
Include in the study all individuals in each selected
cluster
4. Systematic sampling
Selecting every Kth subject from a list of the
members of the population
Advantage
Very easily done
Disadvantages
subgroups
Some members of the population dont have an equal
chance of being included
Systematic sampling (continued)
Selection process
Identify and define the population
Determine the desired sample size
Obtain a list of the population
Determine what K is equal to by dividing the size of
the population by the desired sample size
Start at some random place in the population list
Take every Kth individual on the list
Example, to select a sample of 25 dorm rooms in your
college dorm, makes a list of all the room numbers in the
dorm. For example there are 100 rooms, divide the total
number of rooms (100) by the number of rooms you want
in the sample (25). The answer is 4. This means that you
are going to select every fourth dorm room from the list.
First of all, we have to determine the random starting
point. This step can be done by picking any point on the
table of random numbers, and read across or down until
you come to a number between 1 and 4. This is your
random starting point. For instance, your random starting
point is "3". This means you select dorm room 3 as your
first room, and then every fourth room down the list (3, 7,
11, 15, 19, etc.) until you have 25 rooms selected.
According to Uma Sekaran in Research Method for
Business 4th Edition, Roscoe (1975) proposed the rules of
thumb for determining sample size where sample size
larger than 30 and less than 500 are appropriate for most
research, and the minimum size of sample should be 30%
of the population.
The size of the sample depends on a number of factors and
the researchers have to give the statistically information
before they can get an answer. For example, these
information like (confidence level, standard deviation,
margin of error and population size) to determine the
sample size.
Non-probability samples
(Random): allows a
procedure governed by chance
to select the sample; controls
for sampling bias.
1. Convenience sampling
2. Purposive sampling
3. Quota sampling
1. Convenience sampling:
the process of including whoever happens to
be available at the time
called accidental or haphazard
sampling
disadvantages
difficulty in determining how much
of the effect (dependent variable)
results from the cause (independent
variable)
2. Purposive sampling:
the process whereby the researcher selects a
sample based on experience or knowledge of
the group to be sampled
called judgment sampling
disadvantages
potential for inaccuracy in the researchers
criteria and resulting sample selections
3. Quota sampling
the process whereby a researcher gathers
data from individuals possessing
identified characteristics and quotas
disadvantages
people who are less accessible (more
difficult to contact, more reluctant to
participate) are under-represented
Sampling
in
Qualitative Research
Researchers in qualitative research select
their participants
according to their :
1) characteristics
2) knowledge
It is when the researcher chooses
persons or sites which provide
specific knowledge about the topic of
the study.
1) Maximal Variation Sampling
2) Typical Sampling
3) Theory or Concept Sampling
4) Homogeneous Sampling
5) Critical Sampling
6) Opportunistic Sampling
7) Snowball Sampling
It is when you select individuals that differ on a
certain characteristic. In this strategy you should first
identify the characteristic and then find individuals or
sites which display that characteristic.
It is when you study a person or a site that is
typical to those unfamiliar with the situation.
You can select a typical sample by collecting
demographic data or survey data about all
cases.
It is when you select individuals or sites because they
can help you to generate a theory or specific concepts
within the theory. In this strategy you need a full
understanding of the concept or the theory expected
to discover during the study.
It is when you select certain sites or people
because they possess similar
characteristics. In this strategy, you need to
identify the characteristics and find
individuals or sites that possess it.
It is when you study an exceptional case
represents the central phenomenon in
dramatic terms.
It is used after data collection begins, when you
may find that you need to collect new information
to answer your research questions.
It is when you don't know the best people to
study because of the unfamiliarity of the topic
or the complexity of events. So you ask
participants during interviews to suggest other
individuals to be sampled.
It is the researchers ethical responsibility to
safeguard the story teller by maintaining the
understood purpose of the research
The relationship should be based on trust between
the researcher and participants.
Inform participants of the purpose of the study.
Being respectful of the research site, reciprocity,
using ethical interview practices, maintaining
privacy, and cooperating with participants.
Patton (2002) offered a checklist of general ethical
issues to consider, such as:
reciprocity
assessment of risk
confidentiality,
informed consent
and data access and ownership.
Qualitative researchers must be aware of the
potential for their own emotional turmoil in
processing this information
During the interview process, participants may
disclose sensitive and potentially distressing
information in the course of the interview..
Creswell,J., W. (2012) Educational research:
Planning, Conducting, and Evaluating
Quantitative and Qualitative Research, 4th ed.
Patton,
M.Q. (2002). Qualitative Research and
Evaluation Methods. Thousand Oaks, CA:
Sage.
