Sampling designs

SAMPLING DESIGNS
(Subjects of Research)
Research and Statistics
OLIVER R. GUEVARRA
Instructor

SAMPLING
• technique of getting a representative
portion of a population.
• the term POPULATION is the entire
sum of objects, families, species or
orders of plants or animals.

BENEFITS OF SAMPLING
• 1. Sampling is economical, cheaper and
faster.
• 2. It saves time, money and effort.
• 3. It is more accurate.
• 4. It gives more comprehensive
information.
• It is more effective.

LIMITATIONS OF SAMPLING
• 1. Sample date require more care in
organizing detailed subclassification due
to small number of subjects.
• 2. Complicated sampling plans are
difficult to prepare.
• The characteristic to be observed
infrequently occur in a population, i.e.,
teachers over 30 years of teaching
experience or having outstanding or
unsatisfactory performance.

LIMITATIONS OF SAMPLING
• 4. If sampling plan is not accurately
designed and followed, the results
may give wrong impression.
• Sampling requires expert to conduct
the study in an area. If this is lacking,
the results may be invalid.

PLANNING OF SAMPLING
SURVEY
• 1. State the objectives
• 2. Define the population.
• 3.Choose the sampling individual.
• 4. Find and choose the source of list
of particular individuals involve in the
sample.

SURVEY
• 5. Decide the sampling design to be
used that fits to the study either
scientific or unscientific sampling.
• 6. Determine the sample size by using
the formula,
Ss =NV + [Se2 x (1-p) ] 5.1
NSe + [V2 p(1-p) ]

SURVEY
• where the Ss stands for sample size;
N, population; V, standard value
(2.58) of percent level of probability
with 0.99 reliability level; Se,
sampling error (0.01); and p, largest
possible population.

SURVEY
• 7. Choose the method of determining the
reliability of the sample either test-retest,
split-half, parallel forms, or internal
consistency.
• 8. Test the reliability of the sample in a
pilot institution.
• 9. Interpret the reliability of the sample.
• 10. Select experts to administer the
sample.

Determination of Sample
Size
• Illustration: “Job-Satisfaction and Job
Performance of Private and Public School
Teachers in Iloilo City”
• To have scientific determination of
sample size, consider the steps by using
the formula 5.1 which are as follows:
Ss =NV + [Se2 x (1-p) ] 5.1
NSe + [V2 p(1-p) ]

• Step 1. Determine the total population (N) as
assumed subjects of the study.
• Step 2. Get the value (V= 2.58), Se (0.01), and p
(0.50).
• Step 3. Compute the sample size by using the
formula 5.1. For instance, the total population
(N) is 850; the standard value at 1 percent level
of probability is 2.58 with 99 percent reliability
with sampling error (Se) of 1 percent (1%) or
0.01 and proportion (p) of a target population is
50 percent (50%).

Given
N= 850 Se= 0.01
V= 2.58 p= 0.50
Ss= NV + [ Se2
x (1-p)]
Nse + [V2
x p(1-p)]
Ss= 850 (2.58) + [0.012
x (1-0.50)]
850 (0.01) + [2.582
x 0.50 (1-0.50)]
Ss=
8.5 + 6.6564 x 0.50 (0.50)
Ss=
Ss= 2193.00005
8.5 + 1.16641
Ss= 2193.00005
10.1641
Ss= 215.759393 or 216
2193 + 000005
8.5 + 6.6564 (0.25)
2193 + 0.0001 x 0.50
The sample size of
850 population is
216. This sample,
216, will represent
as subjects of the
study.

Table 5.1
Computed
Sample Size
for Different
population (N)
at 0.01 Level
of Probability
to a Proportion
(0.50)
N Sample Size N Sample Size
100 97 700 208
125 111 725 210
150 122 750 211
175 132 775 212
200 141 825 215
225 148 850 216
250 155 875 217
275 161 900 218
300 166 925 219
325 171 950 220
350 175 1000 221
375 179 1100 224
425 185 1500 232
450 188 1700 235
475 191 2000 238
500 194 2500 242
525 196 3000 244
575 200 3500 246
600 202 4000 248
625 204 4500 249
650 205 5000 250
675 207 6000 251

• Table 5.1 shows that the smaller the
population, the larger is the percentage of
sample size; and the larger the population,
the lesser is the percentage of sample size.
For instance 100 population has 97 sample
size or 97 percent (97%) whereas 1000
population has sample size of 221 or 22.1
percent (22.1%)
N Sample Size N Sample Size
100 97 1000 221

Sampling Designs
• 1. Scientific Sampling- each member
in the population is given an equal
chance of being included in the
sample.
• 2. Non-scientific Sampling- not all the
members in the population are given
equal chance of being included in the
sample.

Types of Scientific Sampling
• 1.Restricted Random Sampling- involves
certain restrictions intended to improve the
validity of the sample. Applicable only
when the population being studied is
homogeneous.RS samples drawn from
homogeneous population are likely to arrive
at accurate values of the population
characteristic.

• 2.Unrestricted Random Sampling- best
random sampling design due to no
restrictions imposed and every member of
the population has equal chance of being
included in the sample.

Restricted or Unrestricted
Random Sampling is chosen in
either of the Two Techniques:
• Lottery Technique- each member in the
population is assigned a number which is
written on a piece of paper.
• For instance, the (N) is 200. There are 200
small paper numbered 1-200. They are
rolled, placed in a box and thoroughly
shuffled.

Restricted or Unrestricted
Random Sampling is chose in
either of the Two Techniques:
• The rolled paper is drawn one at a time.
Rolled papers drawn from 1 to 141 are
included as sample because the sample size
of 200 is 141.The techniques is useful when
the population is small.

Restricted or Unrestricted Random
Sampling is chosen in either of the
Two Techniques:
• Table of Random Numbers- consists of digits
so selected that no systematic relation exists
between any sequence of digits in the table,
regardless of whether the table is read
horizontally, vertically, left or right, or in other
way. To draw ta sample by using table of
random numbers, a technique must be used for
assigning a number to each individual in a
population.

• For instance, the population is 4000. Each
member must be assigned an identifying
number ranging from 1 to 4000. The
identifying number corresponds to the total
number of population, 4000. The sample
size of 4000 is 248. To get a sample of 248
from 4000 (N), a four-digit number are
randomly drawn either horizontally,
vertically, right or left, until 248
individuals are identified as sample.

• 3. Stratified Random Sampling- divides first
the population into two or more strata. For each
stratum, the sample items are drawn at random.
• For instance, an institution has 6000 students. Of
this 6000 students, 4000 are F and 2000 are M.
The sample size of 4000 is 248 and the sample
size of 2000 is 238. Hence, a sample of 248 are
drawn by random technique from the
subpopulation or stratum of females. And 238
samples are drawn from the subpopulation or
stratum of males.

• 3. Systematic Sampling- the
individuals in the population are
arranged in a methodical manner, i.e.,
alphabetical, chronological ( age,
experience or academic rank), and nth
name is selected as sample.

• For instance, there are 900 population in
the study. To select the sample, the 900 are
arranged either alphabetically or
chronologically. They are numbered 1-4
for the first set; 1-4 for the second and so
on till the desired population, 900, are
numbered by 4s. Every 4th of the set is
chosen till the desired sample of 218 is
reached as sample size of 900.

• 5. Multi-stage Sampling- done in different
stages. It can be 2-stage, 3-stage, 4-stage
and many others depending on the number
of sampling to be used. The population
individuals are arranged in the hierarchy of
units, and sampling is done consecutively.

• For instance, in nationwide study, the
regions are selected as first stage;
provinces as second stage;
municipalities as third stage and
barangays as fourth stage. Hence,
there are four stages.

• 6. Cluster Sampling-the population is
grouped into clusters or small units.
• For instance, blocks or districts, in a
municipality or city composed of
population individuals and are chosen
either random sampling or systematic
sampling. This type is beneficial if the
population individuals in blocks or districts
belong to heterogeneous group.

Types of Non-scientific
Sampling
• 1. Purposive Sampling- based on selecting
the individuals as samples according to the
purposes of the researcher as his controls.
An individual is chosen as part of a sample
due to good evidence that he is a
representative of the total population.

Sampling
• 2. Incidental Sampling- “applied to those
samples which are taken because they are
most available.” (Guilford and Fruchter,
1973.) The researcher simply takes the
nearest individuals as subjects of the study
until the sample reaches the desired size.

Sampling
• 3. Quota Sampling- popular in the field of
opinion research because it is done by
merely looking for individuals with the
requisite characteristics. It is usually
prepared by main office with instructions to
field researchers to gather data from
samples that meet the prescribed criteria or
characteristics.

Data-Collection Methods
and Tools
• Instrumentation is a science concerned
with the development of devices for
specific purposes.
• In research, it is the development of
devices to gather data.
• Note: Questionnaire is a tool or device,
while use of questionnaire is a method of
collecting data.

1. Questionnaire
• A set of questions arranged in
sequence and intended to be self-
administered. It is used when a
respondent can read or write.

Questionnaire : Advantages
• 1. It is less expensive.
• 2. Uniformity of questions.
• 3. More confidence of
respondents.

Questionnaire Construction
• A questionnaire should be
attracted, neat and clean with clear
directions. There are three types of
questions that can be used in
questionnaire.

• 1. Open-ended question-allows
the respondent to express himself
on the issue on the question
raised.
• Example: What is your most
pressing problem in growing
vegetables?

• 2. Closed type question- also
called fixed alternative question. It
provides a list of choices from
which the respondent chooses his
answers to the question raised.
• Example: What type of house do
you have?

• 1. Example: What type of house do you
have?
• ( ) Temporary (roof made of cogon or nipa
or bamboo
• ( ) Semi-Permanent (roof made of GI
sheets, posts and walls made of wood
• ( ) Permanent (roof made of GI sheets,
walls and floors made of concrete

• 3. Multiple Response Question-
provides a list of choices from
which the respondent can choose
one or more answers to the
question raised.

• Example: Where do you get capital to
finance your vegetable farm?(Please check
those that apply to you.)
• ( ) Banks ( ) Borrow from relatives
• ( ) Private individual lenders( )
Cooperatives
• Self-financed () Others (Please specify)

• One way to check the clarity and
objectivity of the questionnaire is to
conduct a pre-test or dry run. A pre-test is
the evaluation or testing conducted before
the final distribution of the questionnaire to
enable the researcher to find out which
questions confuse respondents.

• The Cover Letter-help in getting the
cooperation of the respondent and facilitate
retrieval of questionnaires. The
questionnaire should include a carefully
and courteously composed cover letter
with the following information:

• A. date, name and address of the respondent
• B. purpose of the study
• C. request for the cooperation of the
respondent
• D. guarantee of confidentiality and anonymity
of the respondent
• E. statement of gratitude for the participation
of the respondent
• F. Name and signature of researcher

2. Interview
• A face-to-face question and
answer interaction between two
individuals.

Interview: Advantages
• 1. Flexible and adaptable to
individual situations.
• 2. Many people are more willing
to talk than to write.
• 3. It permits probing into the
context and reasons for the
answers to questions.

Interview: Advantages
• 4. Rapport can be established.
• 5. The researcher can explain the
importance of the study.
• 6. Clarificatory questions can be
raised.
• 7. The researcher can detect the
sincerity of the respondents.

Interview: Disadvantages
• 1. Cost of Time
• 2. Money and Effort
• Getting information from an
individual may take one or two
months.
• Costly

Types of Interview
According to Style
• A.Formal Interview-In this type of
interview,the interviewer follows a set of
questions as indicated in the interview
schedule.Answer sheet is provide where the
interviewer writes the answer of the
respondent. It is also called instructured
intervierw.An interview schedule is an
instrument made up of questions almost
similar to a questionnaire accomplished by
the researcher.

Types of Interview
According to Style
.B.Formal Interview-The interviewer uses only interview
guide which contains salient points on which the interview
is centered.It is also called open or unstructured interview.

Some Rules in Interview
1.Assure the respondent of the confidentialy of responses.
2.Avoid leading questions.Leading questions are those that
tend to direct the respondent towards as specific response.
EXAMPLE 1.Have you read about the new school
curriculum? In this question the researcher may get more
‘YES’responses because the questions may imply that if the
respondent answers no, he is not updated of current events
and it is shameful on his part.

EXAMPLE 2: A computer is better than a
typewriter,isn’t?In this question,it seems and it is implied
that the researcher wants the respondent to answer yes. The
question should be worded such that the respondent is free
to give his response, say Which do you think is better,
computer or typewriter?

3.Aviod double-barreled questions. Ask one idea or one
question at a time. In logic, it is called that fallacy of
COMPLEX QUESTION. Examples double-barreled
questions are:

EXAMPLE 1:Are you attending church or chapel? This is
loaded with two questions : whether attending or not, and
whether church or chapel.
EXAMPLE 2:Do you support freedom and the right to bear
arms?This question is composed of two questions, one
about freedom and the other one about bearing arms. But
love of freedom does not always go with bearing arms.

.EXAMPLE 3:Have you stopped cheating and
examinations?(If the answer is yes, it means that the
respondent has been cheating. If the answer is no, it means
that he is still cheating until the time of the interview.)
.

.4.avoid question that call for monologues.
Not this: Will you tell me what your position is regarding
the environment problem in this town? Question like this
will make the respondent talk for an hour or two.(The
researcher can leave his tape recorder running and come
back in an hour when the tape is finished!)The researcher
must pin his question to one clearly defined point, basing on
his research objectives the interview guide.

.Better:What do you consider is the most urgent
environment problem of this town?

.5.Avoid questions that demand knowledge and information
that the respondent does not have. To aviod invalid
responses of the respondent due to lack of information,The
researcher should use FILTER QUESTION. For example,
before asking the respondent about what he thinks about K
to 12 Educational system, the filter question is whether he
knows what K to 12 Educational System is.Another
approach is to explain K to 12 briefly and then ask the
respondent what he thinks of it.

. 6.Be careful with personal or delicate questions that the
respondent may resists.Techniques are needed to obtain
information of a personal, delicate are controversial nature.
As in come and other personal matters later in the interview
after establishing rapport with the respondent.When asking
about something that is socially disapproved,show that
some people believe one way and others believe another
way. Don’t make the respondent disapprove himself.Use
“soft” rather than “hard” expressions.For example,say
“negative reinforecement” or “correction”instead of
“punishment”.

7. Be careful with questions loaded with social
desirabillity.People tend to give response that are socially
desirable, acceptable,of considered good. For example, if a
respondent is ask if he votes , he may be “driven” to answer
yes because everyone is supposed to vote.
A good question is one in which respondents are not led to
merely express socially desirable sentiments (Kerlinger,
1973).

Sampling designs

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