Chapter 6 sampling

EDU702: RESEARCH
METHODOLOGY
THE BASIC OF EDUCATIONAL
RESEARCH

SAMPLING

DEFINITION: SAMPLING
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.

DEFINITION: POPULATION

The larger group from which
individuals are selected to participate.

TARGET VERSUS
ACCESSIBLE POPULATIONS:
1.

The Target Population is the ideal selection of actual
population which researcher really like to generalize:
- is rarely available.
- Researcher’s ideal choice.

2. The Accessible or ‘available’ population is the
population to which a researcher is able to generalize:
- Researcher’s realistic selection

SAMPLING:

1)

2)

RANDOM SAMPLING METHOD
NONRANDOM SAMPLING
METHOD

RANDOM SAMPLING
METHODS
1.
2.
3.
4.

Simple Random Sampling
Stratified Random Sampling
Cluster Random Sampling
Two-Stage Random Sampling

SIMPLE RANDOM SAMPLING
The proces of selecting a sample that
allows induvidual in the defined
population to have an equal and
independent chance of being selected
for the sample.

STEPS IN RANDOM SAMPLING:
1. Identify and define the population.
2. Determine the desired sample size.
3. List all members of the population.
4. Assign all individuals on the list consecutive
number from zero to the required number.
Each individual must have the same number
of digits as each other individual.

5.

Select an arbitrary number in the table of
random numbers.

6. For the selected number, look only at the
number of digits assigned to each population
member.

7.

8.

If the number corresponds to the number
assigned to any of the individual in the
population, then that individual is included
in the sample.
Go to the next number in the column and
repeat step #7 until the desired number of
individuals has been selected for the
sample.

ADVANTAGES OF SIMPLE
RANDOM SAMPLING:


Easy to conduct



Strategy requires minimum
knowledge of the population to be
sampled

DISADVATAGES OF SIMPLE
RANDOM SAMPLING:


Need names of all population members.



May over-represent or under-estimate
sample members.



There is difficulty in reaching all selected
in the sample.

STRATIFIED RANDOM
SAMPLING
The process of selecting a sample
that allows identified subgroups in
the defined population to be
represented in the same proportion
that they exist in the population.

STEPS IN STRATIFIED
SAMPLING:
3. Identify the variable and subgroups (strata)
for which you want to guarantee appropriate,
equal representation.

STEPS IN STRATIFIED RANDOM
SAMPLING
4.

Classify all members of the population as
members of the one identified subgroup.

5.

Randomly select, using a table of random
numbers; an “appropriate” number of
individuals from each of the subgroups,
appropriate meaning an equal number of
individuals.

ADVANTAGES OF STRATIFIED
RANDOM SAMPLING:
 More precise sample.


Can be used both proportions and
stratification sampling.



Sample represents the desired strta.

DISADVANTAGES OF
STRATIFIED RANDOM
SAMPLING:


Need names of all population members.



There is difficulty in reaching all selected
in the sample.



Researcher must have names of all
populations.

CLUSTER SAMPLING
The process of randomly selecting
intact groups, not individuals, within
the defined population sharing
similar characteristics.

STEPS IN CLUSTER RANDOM
SAMPLING:
3. Identify and define a logical cluster.

SAMPLING:
4. List all clusters (or obtain a list) that make up
the population of clusters.
5. Estimate the average number of population
members per cluster.
6. Determine the number of clusters needed by
dividing the sample size by the estimated
size of a cluster.

SAMPLING:
7. Randomly select the needed number of
clusters by using a table of random
numbers.
8. Include in your study all population
members in each selected cluster.

ADVANTAGES OF CLUSTER
RANDOM SAMPLING:
Efficient.
 Researcher does not need nemes of
all population members.
 Reduces travel to site.
 Useful for educational research.


DISADVANTAGES OF CLUSTER
RANDOM SAMPLING:
 Fewer sampling points make it less like
that the sample is representative.

TWO-STAGE RANDOM
SAMPLING
The process of COMBINING Cluster
Random Sampling with an Individual
Random Sampling.

STEPS IN TWO-STAGE RANDOM
SAMPLING:
1.

Select randomly 25 schools from 100 schools
in the district. (Cluster)

ADVANTAGES OF TWO-STAGE
RANDOM SAMPLING:
 Less time-consuming

DEFINITION OF NON RANDOM
SAMPLING


Any sampling method where some
elements of the population have no choice
of selection or where the probability of
selection cannot be accurately determined.

TYPES OF NON RANDOM
SAMPLING METHODS
1.
2.
3.

Systematic Sampling
Convenience Sampling
Purposive Sampling

SYSTEMATIC SAMPLING
The process of selecting individuals
within the defined population from a
list by taking every Kth name.
**K = sampling interval
(sometimes known as the skip)

STEPS IN SYSTEMATIC
SAMPLING:
3. Obtain a list of the population.
4. Determine what K is equal to by dividing the size of
the population by the desired sample size.

Equation
K= N

n
K = sampling interval
N = population size
n = sample size

STEPS IN SYSTEMATIC
SAMPLING:
5. Start at some random place in the population
list. Close your eyes and point your finger to
a name.
6. Starting at that point, take every Kth name on
the list until the desired sample size is
reached.
7. If the end of the list is reached before the
desired sample is reached, go back to the top
of the list.

ADVANTAGES OF SYSTEMATIC
SAMPLING:
 Sample selection is simple

DISADVANTAGES OF
SYSTEMATIC SAMPLING:


All members of the population do not
have an equal chance of being selected.



The Kth person may be related to a
periodical order in the population list,
producing unrepresentativeness in the
sample.

CONVENIENCE SAMPLING
The process of including whoever
happens to be available at the time .
It is also called “accidental” or
“haphazard” sampling.

ADVANTAGE OF CONVENIENCE
SAMPLING:


the availability and the quickness with which
data can be gathered

DISADVANTAGES OF
CONVENIENCE SAMPLING:


Difficulty in determining how much of
the effect (dependent variable) results
from the cause (independent variable)



the risk that the sample might not represent the
population as a whole, and it might be biased by
volunteers

PURPOSIVE SAMPLING
The process whereby the researcher
selects a sample based on experience
or knowledge of the group to be
sampled. It is also called
“judgement” sampling.

ADVANTAGE OF PURPOSIVE
SAMPLING
Useful for situations where you need to reach a
targeted sample quickly and where sampling
for proportionality is not the main concern.

DISADVANTAGES OF
PURPOSIVE SAMPLING:
 Potential for inaccuracy in the
researcher’s criteria and resulting
sample selection.

Identify Types of Sampling
A.
B.
C.
D.
E.
F.
G.

Simple random sampling
Stratified random sampling
Cluster sampling
Two-stage random sampling
Convenience sampling
Purposive sampling
No sampling—entire population is being
studied.



1._____ A researcher is interested in interviewing
a group of New York City police officers. She
goes to the main precinct in Manhattan and asks
for volunteers to be interviewed. The researcher
then pays for the interviews.



all the members of the New York City police
force who do not live in the city. He gets a roster
of the names of all officers on the force, randomly
selects five police stations, and then conducts
interviews of all officers in those stations.

alumni of San Simeon College who graduated
between the years 1990 and 1996. He gets the roster
of the names of these individuals from the alumni
office and mails a questionnaire to everyone on this
roster.
4._____ Another researcher is also interested in
interviewing alumni of San Simeon College who
graduated between the years 1990 and 1996. He gets
the roster of the names of these individuals from the
alumni office, selects the names of 100 individuals
who graduated during these years using a table of
random numbers, and then mails a questionnaire to
everyone selected.



5._____ A researcher is interested in identifying the
attitudes of the physicians who work for Keyser
Hospital toward the Republican plan for health care.
She obtains a list of all the Keyser Medical Centers in
Southern California and randomly selects ten of these
Centers. Then she obtains a list of all the physicians
at these Centers and randomly selects eight
physicians from each Center to interview.



6._____ A graduate student enrolled in the Marriage
and Family Counseling Program at Daytona
University is interested in determining how other
graduate students feel about the program. He
interviews all of the students he has access to on a
given Monday night when he takes one of his
counseling courses.



7._____ A student enrolled in the Hotel and
Restaurant Management School at Colorado
State is researching the best restaurants in
Denver based on the opinions of food critics.
She begins by asking her advisor who refers
the student to four food critics who have
written extensively on the subject and whom
the student then contacts to interview for her
study.

Chapter 6 sampling

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Chapter 6 sampling