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Sampling What, why & how
Dr Girija P. Kartha
SamplingWhat, why & how
What is sampling?“Sampling is the act, process, or technique of selecting a suitable sample, or a representative part of a population for the purpose of determining parameters or characteristics of the whole population.”
SamplingWhat, why & how
Why sampling?
Sampling is done because you usually
cannot gather data from the entire population.
SAMPLING…….
TARGET POPULATION
STUDY POPULATION
SAMPLE SAMPLE
SamplingWhat, why & how
HOW ?
SAMPLE SELCTION
SAMPLING METHODS
SAMPLE SIZE
SamplingWhat, why & how
SAMPLE SELCTION
Key factor in research design.
determine whether research questions will be answered
clearly identify the population of interest for the specific research
question.
SamplingWhat, why & how
SAMPLING METHODS
MAINLY TWO METHODS:
PROBABILITY SAMPLING
NON-PROBABILITY SAMPLING
SAMPLING METHODS
PROBABILITY vs NON-PROBABLILITY SAMPLING
PROBABILITY vs NON-PROBABLILITY SAMPLING
BASIS FOR COMPARISON
PROBABILITY SAMPLING
NON-PROBABILITY SAMPLING
Meaning
The subjects of the population
get an equal opportunity to be selected as a representative
sample.
Subjects chosen by
researcher. Hence, who
will be selected can
not be predicted
PROBABILITY vs NON-PROBABLILITY SAMPLING
BASIS FOR COMPARISON
PROBABILITY SAMPLING
NON-PROBABILITY SAMPLING
Alternately known as
Non-random sampling
Random sampling
Basis of selection
Randomly Arbitrarily
Research Conclusive Exploratory
PROBABILITY vs NON-PROBABLILITY SAMPLING
BASIS FOR COMPARISON
PROBABILITY SAMPLING
NON-PROBABILITY SAMPLING
Result BiasedUnbiased
Method Objective Subjective
Hypothesis Tested Generated
SAMPLING TECHNIQUES
Simple random sampling
o Basic, most commonly used and easiest method.
o Each unit of the population has an equal chance of
inclusion in the sample.
o Applicable when population is small,
homogeneous and readily available.
Probability Sampling:
Simple
Random
Mainly two approaches are used…
• Lottery method
• Table of random number
Simple random sampling
Simple
Random
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 37 75 10 49 98 66 03 86 34 80 98 44 22 22 45 83 53 86 23 51
2 50 91 56 41 52 82 98 11 57 96 27 10 27 16 35 34 47 01 36 08
3 99 14 23 50 21 01 03 25 79 07 80 54 55 41 12 15 15 03 68 56
4 70 72 01 00 33 25 19 16 23 58 03 78 47 43 77 88 15 02 55 67
5 18 46 06 49 47 32 58 08 75 29 63 66 89 09 22 35 97 74 30 80
6 65 76 34 11 33 60 95 03 53 72 06 78 28 14 51 78 76 45 26 45
7 83 76 95 25 70 60 13 32 52 11 87 38 49 01 82 84 99 02 64 00
8 58 90 07 84 20 98 57 93 36 65 10 71 83 93 42 46 34 61 44 01
9 54 74 67 11 15 78 21 96 43 14 11 22 74 17 02 54 51 78 76 76
10 56 81 92 73 40 07 20 05 26 63 57 86 48 51 59 15 46 09 75 64
11 34 99 06 21 22 38 22 32 85 26 37 00 62 27 74 46 02 61 59 81
12 02 26 92 27 95 87 59 38 18 30 95 38 36 78 23 20 19 65 48 50
13 43 04 25 36 00 45 73 80 02 61 31 10 06 72 39 02 00 47 06 98
14 92 56 51 22 11 06 86 88 77 86 59 57 66 13 82 33 97 21 31 61
15 67 42 43 26 20 60 84 18 68 48 85 00 00 48 35 48 57 63 38 84
`
Tab
le o
f ran
dom
nu
mb
er
oRandom Numbers can be generated in excel by clicking of the formula
Systematic Random Sampling
o Applicable when the population is large, scattered and not
homogeneous.
o There is an equal interval between each selected unit.
o Population size – N, desired sample size- n,
Sampling interval K=N/n
o Randomly select a number and then every Kth element
thereafter.
Systematic Random Sampling
• Simple & convenient.
• Easier to implement in the field.
• The time and labor is relatively
small.
Systematic
Stratified Random Sampling
• Applicable when population is not homogeneous.
• The population is first devided into homogeneous
groups (age, sex, area, classes) called Strata.
• Then a simple random sample is selected from each
stratum.
Stratified
Stratified random sampling
◼It gives more representative sample.
◼Characteristics of each stratum can
be estimated and comparisons made.
◼It gives greater accuracy.
Stratified
MULTISTAGE SAMPLING
20
• The selection is done in stages. Cluster sampling is an
example of multistage sampling.
• First stage, random number of districts chosen in all states.
• Followed by random number of talukas, villages.
• Then third stage units will be houses.
• All ultimate units (houses) selected at last step are
surveyed.
Multiphasic sampling
• Part of the information is collected from whole population and part from sub samples.
• Eg. 20 fever cases clinical examination & blood test - high ESR - Widal, MP etc. –Those found negative again subjected to other tests.
Cluster Sampling• A cluster is a randomly selected group.
• Applicable when units of population are natural groups or clusters (villages, wards, blocks, factories, schools, etc.).
• From the chosen clusters, 30 in
number, the entire population
is surveyed.
Cluster
Cluster Sampling
In cluster sampling, instead of selecting all the subjects from the entire population right off, the researcher takes several steps in gathering his sample population. Cluster
Non-
probability
Quota
Judgment
SnowballConvenience
Non-Probability Sampling Methods
◼ Convenience Sample
◼ The sampling procedure used to obtain those units or people most conveniently available
Convenience/Purposive Sampling
o Subjects selected because it is easy to access them.
o This method is also called CHUNK.
o Generally used for making pilot studies.
o Biases are maximum.
o Results obtained are unsatisfactoryin terms of drawing conclusions.
Convenience
Judgment Sampling
The choice of study units depends exclusively on the judgment of theinvestigators.
based on some character of sample members to serve a purposeo
Judgment
Quota Sampling
o In this, the population is divided into quotas according to some specific characteristics
(age, sex, religion, etc.) represented to the exact extent that the investigator desires
No. of study units selected within each quotadepend on personal judgment.
.
◼ Snowball Technique
• When the target population is hidden and or
hard to reach
such as commercial sex workers, drug addicts or a patients of a rare disease or happenings in a controversial event.
Snowball
◼ Snowball sampling
◼ Initial respondents chosen by probability or non-probability methods,
◼ Then additional respondents are obtained by information provided by the initial respondents
Snowball
Person 1
Friend/contact 1
contacts his/her
own friends/co
ntacts/
Friend/contact 2
contacts his/her own
friends/contacts/
Friend/contact 3
contacts his/her
own friends/co
ntacts/4 5 6 7 8 9 10
`11 12
RESEARCHERRESEARCHER HAS 3 CONTACTS
Each 3 contacts has another 3 contacts.
SNOWBALL SAMPLING
Snowball
GROUP ACTIVITYLet us try some questions…
a) Which sampling method you will choose to identify the alcoholics in a city ?.
b) I want to study on the thought process of students who prefer to do post graduation in your class Which method of sampling would you choose?
c) In a TB survey, MT was done in all cases. In all MT +ve cases X-ray chest was done .
In all X-ray +ve cases sputum examination was done.Which survey method was used?
GROUP ACTIVITYGROUP – A
Q.1 There are 100 employees in an organization. The HR deptt. wants to select employees for a team building activity. What sampling method will be ideal ?
Q. 2 A researcher wants to know the views of the people who attended a dance program about certain aspects of the program. Which method would be chosen?
GROUP ACTIVITYGROUP – B
Q.3 Government of India wants to evaluate effectiveness of immunization program in Gujarat. Which method can be chosen?
Q. 4 A researcher wants to know the reasons of choosing ethical hacking as a profession. How will you choose your sample?
GROUP ACTIVITYGROUP – C
Q.5 To evaluate the popularity of use of City Metro as a transportation system as per the different income groups on a randomly selected day. What will be the ideal method of choosing samples?
Q. 6 To analyse the views of the population of a village about mobile phone usage which sampling method would be ideal?
GROUP ACTIVITYGROUP – D
Q.7 A researcher wants to study the prevalence of depression among the female population of Surendranagar. Which method of sampling can be chosen?
Q. 8 A researcher wants to know the effectiveness of the family planning program by choosing 500 adults from 5 villages with total population of 10000 each. Which is ideal sampling method?
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