Sampling design

Sample &Sampling
Design
DR.G.SINGARAVELU
Associate Professor
UGC-ASC
BHARATHIAR UNIVERSITY
COIMBATORE

DEFINITIONS
Population-totality of the objects or
individuals regarding inferences are
made in a sampling study.

Sample-smaller representation of a
large whole.

Sampling- is a process of selecting a
subset of randomised number of the
members of the population of a study

Sampling frame /Source list -complete list of all the
members/ units of the population from which each
sampling unit
Sample design / sample plan-is a definite plan for obtaining
a sample from a given population.

Sampling unit-is a geographical one (state,district)

Sample size-number of items selected for the study

Sampling Error-is the difference between population value
and sample value.

Sampling distribution-is the relative frequency distribution of
samples.

CENSUS/SAMPLING
Census-collection of data from
whole population.

Sampling is taking any portion of
a population or universe as
representative of that
population.

Sampling method has been using
in social science research since
1754 by A.L.BOWLEY

Indispensable of sampling in
Research
Saves lot of time
Provides accuracy
Controls unlimited data
Studies individual
Reduces cost
Gives greater speed /helps to complete in
stipulated time
Assists to collect intensive and exhaustive data
Organises conveniences

Steps in Sampling Process /
Procedures
Define the population (element,units,extent and
time)
Specify sampling frame(Telephone directory)
Specify sampling unit (retailers, our
product,students,unemployed)
Specify sampling method/technique
Determine sampling size
Specify sampling size-(optimum sample)
Specify sampling plan
Select the sample

PRINCIPLES OF SAMPLING
Two important principles
Principles of Statistical Regularity-random
(sufficient representative of the sample),
Principles of Large Numbers-(steadiness ,
stability and consistency)
Principles are referred to as the laws of
sampling

Good sampling
The sample should be true representative of
universe.
No bias in selecting sample
Quality of the sample should be same
Regulating conditions should be same for all
individual
Sampling needs to be adequate
Estimate the sampling error
Sample study should be applicable to all items

Preparing a sampling design
Type of universe (set of objects)
Finite/Non-finite
Sampling unit (district,school,products)
Sampling frame
Sampling size
Sampling technique

Methods of sampling
Bloomers and Lindquist
Probability Non Probability
Random/simple Quota
Stratified random Purposive
Cluster Accidental
Systematic Incidental
Multistage
Proportionate Snow ball

Probability
Probability sampling technique is one
in which every unit in the population has a
chance of being selected in the sample
This probability can be accurately
determined.

Nonprobability sampling
Nonprobability sampling is any sampling
method where some elements of the population
have no chance of selection (these are
sometimes referred to as 'out of
coverage'/'undercovered'), or where the
probability of selection can't be accurately
determined.
It involves the selection of elements based on
assumptions regarding the population of interest,
which forms the criteria for selection.
The selection of elements is non random
.

Simple random sampling
In a simple random sample ('SRS') of a given size, all
such subsets of the frame are given an equal probability.
Method of chance selection. Lottery method,Tippet’s
table, Kendall and Babington smith, Fisher and Yate’s
numbers.
Simple random sampling with replacement:- equal
probability selection of each unit=1/N (Monte-Carlo
simulation)
Simple random without replacement -varying probability
selection of each. First unit=1/N , Second unit=1/N-1,
Probality of selection of the nth unit=1/N-(n-1)(Monte-
Carlo simulation

Systematic
Systematic sampling involves a random
start and then proceeds with the selection
of every kth element from then onwards.
In this case, k=(population size/sample
size).
It is important that the starting point is not
automatically the first in the list, but is
instead randomly chosen from within the
first to the kth element in the list
Sampling interval width=I=N/n=800/40=20

Stratified or Mixed sampling
Where the population embraces a number of
distinct categories, the frame can be organized
by these categories into separate "strata." Each
stratum is then sampled as an independent sub-
population, out of which individual elements can
be randomly selected .(homogenous group)

Two types-Proportionate (equal number of unit
from each stratum proportion to size of the
strata) and Disproportionate (not equal number
of unit from each stratum proportion to size of
the strata)

Cluster sampling
Cluster sampling is an example of 'two-
stage sampling' or 'multistage sampling/
Multi phase sampling'
in the first stage a sample of areas is
chosen
in the second stage a sample of
respondents within those areas is
selected.(several stages)- State level,Dist
level,Village level,Hosehold level.

Cluster Sampling

This stepwise process is useful for those who
know little about the population they’re studying.
First, the researcher would divide the population
into clusters (usually geographic boundaries).
Then, the researcher randomly samples the
clusters.
Finally, the researcher must measure all units
within the sampled clusters.
Researchers use this method when economy of
administration is important.

Sequential sampling
Single sampling

Double sampling

Multiple sampling

Non probability
Non probability sampling does not
involve random selection and
probability sampling does .

Multistage sampling
Multistage sampling is a complex form of cluster
sampling in which two or more levels of units are
embedded one in the other.
The first stage consists of constructing the clusters that
will be used to sample frame.
In the second stage, a sample of primary units is
randomly selected from each cluster (rather than using
all units contained in all selected clusters).
In following stages, in each of those selected clusters,
additional samples of units are selected and so on.
All ultimate units (individuals, for instance) selected at
the last step of this procedure are surveyed.

Purposive/Judgment Sampling

In purposive sampling, selecting sample
with a purpose in mind
Purposive sampling can be very useful for
situations where we need to reach a
targeted sample quickly and where
sampling for proportionality is not the
primary concern.
It is for pilot study
Questions / questionnaires may be tested.

Quota sampling

Quota sampling, the population is first
segmented into mutually exclusive sub-groups,
just as in stratified sampling.
Then judgment is used to select the subjects or
units from each segment based on a specified
proportion. For example, an interviewer may be
told to sample 200 females and 300 males
between the age of 45 and 60.
Proportional quota sampling
Nonproportional quota sampling
It is very popular for market survey and opinion
poll.

Snowball Sampling

Identifying someone who meets the
criteria for inclusion in the study.
Snowball sampling is especially useful
when we are trying to reach populations
that are inaccessible or hard to find
This method would hardly lead to
representative samples
Intially certain members and add few
members latter

Convenience sampling
Convenience sampling (sometimes
known as grab or opportunity sampling)
is a type of nonprobability sampling which
involves the sample being drawn from that
part of the population which is close to
hand

Accidental Sampling
The researcher can select any sample in
any place, can collect the data from
pedestrian also.
It can be used for exploratory studies
It has sample error.
It has less accuracy

Combination of Probability sampling
and Non Probability sampling
If sampling is carried out in series of
stages, it is possible to combine
probability and non-probability sampling in
one design
Users of particular product in one street
for the particular group of people.
Utility of the particular product in the town.

Sampling Errors
The errors which arise due to the use of
sampling surveys are known as the sampling
errors.
Two types of sampling errors-Biased Errors,
Unbiased Errors
Biased Errors-Which arise due to selection of
sampling techniques.-size of the sample
Unbiased Errors / Random sampling errors-arise
due to chance differences between the members
of the population included in the sample and not
included.

Methods of reducing Sampling
Errors
Specific problem selection
Systematic documentation of related
research
Effective enumeration
Effective pre testing
Controlling methodological bias
Selection of appropriate sampling
techniques.

Non-sampling Errors
Non-sampling errors refers to biases and mistakes in
selection of sample.
CAUSES FOR NON-SAMPLING ERRORS
Sampling operations
Inadequate of response
Misunderstanding the concept
Lack of knowledge
Concealment of the truth.
Loaded questions
Processing errors
Sample size

Factors related to Sample size
The nature of population
Complexity of tabulation
Problems relating to collection of data
Selection of sampling techniques
Limitation of accuracy
Calculating sample size=(SZ / T)2
S-preliminary SD of the universe
Z-number of standard errors
T-errors to be tolerated

Sampling design

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