33 Sampling
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SamplingSampling
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SamplingSampling
Universe
Population
Sample
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Census and samplingCensus and sampling
Census : Obtaining information fromthe entire population
Sample: only from a smallpercentage of population
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Sampling Design and ProceduresSampling Design and Procedures
Population
Sampling Frame
Sampling units Sample size
Sampling techniques
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Example: Tupperware is conducting aExample: Tupperware is conducting asurvey in Bangaloresurvey in Bangalore
Population: All women in Bangalore
Sampling Frame: credit card holders
Sampling units: each Woman abovemonthly income of Rs. 25000
Sampling size; 1000
Sampling technique: random
Sampling execution: How many, atwhat time, what questionnaire
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Probability and NonProbability and NonProbability SamplingProbability Sampling
Probability Sampling
Simple Random sampling
Systematic Stratified
Cluster
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NonNon--ProbabilityProbability
Convenience
Judgmental
Quota Snowball
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Probability samplingProbability sampling
Probability: Sampling units areselected by chance
Each element of the population has afixed probabilistic chance of beingselected
Non Probabilistic: Relies on the
personal judgment of the researcherrather than chance to select sampleelements
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Simple RandomSimple Random
Each element in the population has aequal probability of selection
Generate the random numbersthrough computers, lottery, etc
Holds good for a homogenouspopulation
Ex; quality check for packages forproducts arriving through shipments
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Simple randomSimple random
One of the easiest means ofsampling
Difficult to construct the samplingframe from where u have to drawthe sample
Can result with lower precession with
larger standard error May not represent the population
truely
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Systematic samplingSystematic sampling
Random starting point and then pickthe ith unit in succession.
ith
unit is the sampling intervaldecided by = population size/samplesize
Say i= 10000/1000 = 10
If8 is randomly generated then 8,18, 28, 38, etc
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Here sampling frame should be anorganized one.
Ordering of elements is important.
Say ordering of credit cardcustomers based on outstandingbalance. This would increase therepresentativeness of the sample.
This is less costly and easier thanSRS
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Stratified samplingStratified sampling
Sampling process wherein you classify thepopulation into subpopulation or strata.
The strata should be mutually exclusive.
Elements are selected from each stratumby a random procedure usually SRS
Suitable when the population is
heterogenous and you can representdifferent sections of population.
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Classification should be such that unitswithin the strata are as homogenousas possible and units and elements
across different strata should be asheterogenous as possible.
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Cluster samplingCluster sampling
A sample of subpopulation is drawnrandomly.
It is cheaper than stratifiedsampling.
Cluster is usually a geographicalarea.
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NonNon--ProbabilityProbability
Convenience sampling
Collect information from friends or
colleagues or people in your streetOn the street interviews or on the mall
interviews
Least expensive of all, easilyaccessible, easy to measure and co-operative.
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ConvenienceConvenience
Suitable for exploratory research andnot in-depth studies
Good for generating ideas insight orhypothesis.
Cannot draw inferences from suchsampling.
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Judgmental samplingJudgmental sampling
Sample is chosen based on thejudgment of the researcher.
Low cost, convenient and quick. Extension of this is quota sampling
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Quota SamplingQuota Sampling
Viewed as two stage restrictedjudgment sampling
Divide the population into controlgroups or quotas based on yourjudgement.
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Quotas are assigned in such a waythat the proportion of sampleelements possessing the control
characteristics is the same as theproportion of population elementswith these characteristics
Or the composition of sample is thesame as the composition ofpopulation
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Study the readership of TheStudy the readership of TheHinduHindu
Controlcharacteristics
Populationcomposition
Samplecomposition
Sample size
Sex Sample of1000
Male 52 52 520
Female 48 48 480
Age
18-30 25 25 250
31-41 35 35 350
45-60 20 20 200
60 and above 20 20 200
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Snowball samplingSnowball sampling
Reference sampling
An initial group of respondents is
selected usually at random Then they are asked to refer to other
contacts
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Sampling ErrorSampling Error
Sample mean is not equal to thepopulation mean
Occurs due to wrong representationof the sample
Due to wrong sample size ortechnique
Reduce it by increasing the samplesize.
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Non Sampling errorNon Sampling error
Known as systematic errors
Due to response error.
Measurement error Problem with the questionnaire
design.
Selection of faulty sampling frame
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Internet SamplingInternet Sampling
One of the popular means of datacollection
Useful if you have a responsible setof respondents.
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Internet samplingInternet sampling
Systematic sampling of websitevisitors
Volunteer panels Opt in lists
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Website visitorsWebsite visitors
Po-up applications
Invite an nth visitor to respond to yoursurvey
You need to have a web application tocreate a pop-up window
Web application companies. Like
Zoomrang.com First take the consent and then if obtained
open the survey page.
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Volunteer PanelsVolunteer Panels
Group of respondents who haveagreed to co-operate withresearchers future e-mail or on-linesurveys
The respondents are recruitedthrough adds or through agencies
The respondents are compensated
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OptOpt--in listsin lists
Sending e-mails to the list of peopleopted for the survey
It might be otherwise considered asjunk mail.
Becomes important to ensure thatmails are not sent to users who havenot opted for participation in onlinesurveys.
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Sample size determinationSample size determination
Ideal sample size for good results
Sample size would differ based on
the nature of the population and sizeof population
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Sample size would dependSample size would dependonon
Standard deviation of the population
Acceptable level of sampling error
Confidence level
If the population has less deviation small sample is sufficient
Acceptable level of sampling error: what isthe precession that u need
Confidence level
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N = (ZS/E)2
Z = standardised value corresponding to a
confidence level S = sample standard deviation
E = acceptable level of sampling error
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On an average how much money do
people spend on auto rickshaw?
n = ((1.96*50)/5)2
n = 384
Suppose u increase E from 5 to 7then n = 196
Decrease E from 5 to 4 then n =600