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

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33 Sampling