Research Sampling
Session 13: Research Sampling
Total Session Time: 120 minutes + 240 minutes assignment
Prerequisites
None
Learning Tasks
By the end of this session students are expected to be able to:
Define the common terms used in sampling
Identify the population(s) to be studied
Describe common methods of sampling
Explain reasons for sampling
Describe source of bias in sampling that should be avoided
Resources Needed
Flip charts, marker pens, and masking tape
Black/white board and chalk/whiteboard markers
Computer and LCD Projector
Handout 13.1: Types of sampling methods and when to use it
SESSION OVERVIEW
Activity/
Step Time Content
Method
1 05 minutes Presentation Introduction, Learning Tasks
2 20 minutes Presentation Definition of the Common Terms Used in Sampling
20 minutes Presentation Identification of the Population(s) to be Studied
3
Buzzing
35 minutes Presentation
4 Group Common Methods of Sampling
discussion
5 10 minutes Presentation Reasons for Sampling
6 15 minutes Presentation Source of Bias in Sampling that Should be Avoided
7 05 minutes Presentation Key Points
8 05 minutes Presentation Evaluation
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9 05 minutes Presentation Assignment
SESSION CONTENTS
STEP1: Presentation of Session Title and Learning Tasks (5 minutes)
READ or ASK students to read the learning tasks and clarify
ASK students if they have any questions before continuing
STEP 2: Definition of Common Terms Used in Sampling (20 minutes)
o For studies which involve only small numbers of people all of them can be included
in the investigation.
o For research which focuses on a large population, for practical reasons, it is only
possible to include some of its members in the investigation. It is important to draw a
sample from the total population.
Sampling unit
o A single section selected to research and gather statistics of the whole.
o For example, when studying a group of college students, a single student could be a
sampling unit.
o Unit of selection in the sampling process, e.g. Person, a school, a household, etc.
Sampling frame
o a list of units from which a sample is to be picked
Sampling fraction (sampling ratio)
o a proportion of sampling units to be picked from a specified sampling frame
number of units in sampling frame
Sampling interval
o An interval at which units are picked from a sampling frame when systematic
sampling is done.
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STEP 3: Identifying the Population (s) to be Studied (20 minutes)
Activity: Buzzing (5 minutes)
ASK students to pair up and buzz on the following question for 2 minutes
‘What is study population?
ALLOW few pairs to respond and let other pairs to add on points not mentioned
WRITE their response on the flip chart/board
CLARIFY and SUMMARIZE by using the content below
Study population
o Collective of study units for which the values of the varieties of interest could
possibly be determined.
o In identifying the population(s) to be studied we must consider the following questions:
What is the group of people (STUDY POPULATION) we are interested in from
which we want to draw a sample?
How many people do we need in our sample?
How will these people be selected?
Sample is a subset of study population selected to participate in the research when
whole study population cannot be reached
o The study population has to be clearly defined (for example, according to age, sex, and
residence.) Otherwise we cannot do the sampling
o Each study population consists of STUDY UNITS. The way we define our study
population and our study unit depends on the problem we want to investigate and on the
objectives of the study
o If researchers want to draw conclusions which are valid for the whole study
population, which requires a quantitative study design, they should take care to draw a
sample in such a way that it is representative of that population.
o A representative sample has all the important characteristics of the population from
which it is drawn
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Figure 1: Relationship between population and sample
STEP 4: Common Methods of Sampling (35 minutes)
Activity: Small Group Discussion (20 minutes)
DIVIDE students into small manageable groups
ASK students to discuss on the following question
ALLOW students to discuss for 10 minutes
ALLOW few groups to present and the rest to add points not mentioned
CLARIFY and SUMMARIZE by using the contents below
The common methods of sampling are:
Two types of sampling probability and non-probability
o Non-probability
o Probability (random sampling)
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o Involves using random selection procedures, to ensure that each unit of the sample is
chosen on the basis of chance.
o All units of the study population should have an equal, or at least a known chance of
being included in the sample.
o Requires listing of all study units exist or can be compiled. This listing is called the
sampling frame.
o Simple random sampling
o Systematic sampling
o Stratified sampling
o Cluster sampling
o Multistage sampling
To select a simple random sample, you need to:
o Make or search for an existing numbered list of all the units in the population from
which you want to draw a sample (sampling frame).
o Decide on the size of the sample.
o Select required number of sampling units, using a lottery method.
o For example, a simple random sample of 50 students is to be selected from a school of
250 students. Using a list of all 250 students, each student is given a number (1 to
250), and these numbers are written on small pieces of paper. All the 250 papers are
put in a box, after which the box is shaken vigorously, to ensure randomization. Then
50 papers are taken out of the box, and the numbers are recorded. The students
belonging to these numbers will constitute the sample.
o Individuals are chosen at regular intervals (for example every fifth) from the sampling
frame.
o Ideally a number to tell where to start selecting individuals from the list is randomly
selected.
For example, a systematic sample is to be selected from 1200 students of a school. The
sample size selected is 100. The sampling fraction is:
The sampling interval is therefore 12.
The number of the first student to be included in the sample is chosen randomly
o If it is important that the sample includes representative study units of small groups
with specific characteristics, then sampling frame must be divided into groups, or
strata, according to these characteristics.
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o For example, residents from urban and rural areas, or different religious or ethnic
group
o Random or systematic samples of a pre-determined size will have to be obtained from
each group (stratum).
o The selection of groups of study units (clusters) instead of the selection of study units
individually.
o Clusters are often geographic units (e.g. districts, villages) or organizational units (e.g.
clinics, training groups).
o A multi-stage sampling procedure is carried out in phases and it usually involves
more than one sampling method. Example selecting one region out of 26 then select 4
councils out of 7 in the selected region. In the 4 selected councils, the researcher
selects 3 wards out of 8 then two villages are selected from the 3 wards. The selection
method in each stage could be random or systematic.
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o Unequal chance of being included in the sample (nonrandom)
o It is a sampling process in which the samples are selected for specific purpose with a
predetermined basis for of selection
o Purposeful sampling strategies for qualitative studies
o Extreme case sampling
o Maximum variation sampling
o Homogeneous sampling
o Critical case sampling
o Snowball or chain sampling
o Sample Size in Qualitative Studies
There are no fixed rules for sample size in qualitative research.
The size of the sample depends on what you try to find out, and from what
different informants or perspectives
o If you want to explore how you can involve mothers in your HC catchment area
you try to find that out. For example:
effectively in early detection and treatment of pneumonia, you might decide to
conduct some FGDs to assess mothers‘ knowledge, attitudes and practices with
respect to antimalarials.
You could start with two FGDs among lowly educated mothers and two among
mothers with more education (who usually are of higher socio-economic status).
o If research objective is more complex e.g., attitudes of males and females towards
family planning, and has policy implications for a larger area, your sample will be
bigger. You might start with four FGDs, two among males and two among females,
subdivided according to socio-economic status.
possible, but not determined.
o The desirable sample size depends on the expected variation in the data (of the most
important variables).
o The more varied the data are the larger the sample size needed to attain the desired
level of accuracy.
o After conducting FGDs and in-depth interviews in the study on attitudes of men and
women towards family planning you might decide to conduct a bigger survey.
o If the exploratory study revealed that age and education appear to be important factors
determining FP use, compare FP use in groups with different levels of education and
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of different ages.
o If you split each of these variables up in three categories, and you select four
categories of informants (male users/ spouses of female users; female users; male
non-users, female non-users) you would have 12 cells in each table. In order to obtain
5-10 answers per cell you would require 60-120 informants in each research area.
feasible.
Refer students to Handout 13.1: Types of sampling and when to use it
STEP 5: Reasons for Sampling (15 minutes)
Reasons for sampling
adequate to reach everyone in the population
census study and produces results at a relatively faster speed.
conducted by trained and experienced investigators.
study
information concerning some characteristic of the population
STEP 6: Sources of Bias in Sampling that Should be Avoided (15 minutes)
results of the Study can be a consequence of improper sampling procedures, which result
in the sample not being representative of the study population.
known source is non-response.
o The bigger the non-response rate, the more necessary it becomes to take remedial
action.
o It is important in any study to mention the non-response rate and to honestly discuss
whether and how the non-response might have influenced the results.
o Studying volunteers only: The fact that volunteers are motivated to participate in the
study may mean that they are also different from the study population on the factors
being studied.
o It is better to avoid using non-random selection procedures that introduce such an
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element of choice.
systematically from people seeking alternative treatments.
duration are more likely to be missed. This may mean missing fatal cases, cases with
short illness episodes and mild cases.
characteristics in different seasons of the year. For this reason, data should be collected on
the prevalence and distribution of malnutrition in a community during all seasons rather
than just at one point in time.
o Data collection tools (including written introductions for the interviewers to use with
potential respondents) should be pre-tested. If necessary, adjustments should be made
to ensure better co-operation.
o If non-response is due to absence of the subjects, follow-up of non-respondents may
be considered.
o If non-response is due to refusal to co-operate, an extra, separate study of non-
respondents may be considered in order to identify to what extent they differ from
respondents.
o Another strategy is to include additional people in the sample, so that non-respondents
who were absent during data collection can be replaced. This can only be justified if
their absence was very unlikely to be related to the topic being studied.
o The bigger the non-response rate, the more necessary it becomes to take remedial
action.
o It is important in any study to mention the non-response rate and to honestly discuss
whether and how the non-response might have influenced the results.
STEP 7: Key Points (5 minutes)
Sampling is the process of selecting a number of study units from a defined study
population
BIAS in sampling is a systematic error in sampling procedures which leads to a distortion
in the results of the study
STEP 8: Evaluation (5 minutes)
What are reasons for sampling?
What are source of bias in sampling that should be avoided?
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STEP 9: Assignment (5 minutes
Activity: Take Home Individual Assignment (05 minutes)
ASK each student to select to identify population to be studied and the sampling technique
for a research proposal being developed
ALLOCATE time for student to do the assignment and submit
REFER students to recommended reference
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References
Hardon A, Boonmongkon P, and Streefland P. et al (2001). Applied Health research,
Anthropology of health and health care, (3rd Ed) Amsterdam, The Netherlands: Het
Spinhuis Publishers
Beaglehole R, Bonita R and Kjellstrom (1993) Basic epidemiology: Geneva, Switzerland:
World Health Organization,
Kothari C.R (1985). Research Methodology – Methods and techniques, (2nd ed); New Delhi,
India; Wiley Eastern Limited
Stewart A (2001). Basic Statistics and epidemiology, A practical guide,; London, United
Kingdom: Radcliffe Medical Press,
Varkevisser, C. M, Pathmanathan, I and Brownlee, A (1991). Designing and Conducting
Health Systems Research Projects, Vol. 2 Part I: Ottawa, Canada: IDRC
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Handout 13.1: Types of sampling and when to use it
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