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Sampling Fundamentals
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Steps intheSamplingProcess
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Sample or Census
Population
(Census)
Sample
An aggregate of all the elementsthat share some common set of
characteristics and that comprisethe universe for the purpose of the
marketing research problem.
A subgroup of the populationselected for
participation inthe study .
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Sampling Terms
Census - count of the populationSample - subset of the population
Sample Unit/Element - basic level of investigation i.e.:
Household, Person, BusinessSampling Frame - a master list of the all sample unitswithin the population
i.e.: Telephone Book, List of registered businesses within anindustry
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Sample or CensusWhen is a censusappropriate?
population is small
information required from
everyone in populationcost of incorrect decisionis high
sampling errors are high
When is a sampleappropriate?
population is large
quick decision is needed
cost is an issue
improved quality
homogeneous population
census is not possible
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A precise statement of who should and should not beincluded in the sample
Do we only want to interview those that participate in sportat the professional level? Should we ignore amateur sportspeople?
Elements
An element about which or from which the information is desired.
eg. Respondent - male, female, over 18, main grocery buyer,decision maker for a new product
The Target Population
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Selecting a Sampling Procedure
Probability sample All population members have a knownprobability of being selected in thesample
Nonprobability sampleProbability is unknown, based on
convenience/judgement
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Probability Sampling
Simple random sampling
Systematic sampling
Stratified samplingCluster sampling
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Simple Random Sampling
Researcher uses:Random number tables
Random digit dialing
Some other randomselection procedure
Guarantees that eachmember of the populationhas an equal chance of selection
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Table of random numbers
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Systematic Sampling
Researcher selects astarting point (usually bySRS)
Then a constant skip intervalis used to select every other member
The skip interval used mustcover the entire pop
regardless of starting pointIs more efficient than SRS
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Tennis magazine conducted a mail survey of its subscribers to gain a better understanding of its market. Systematic sampling was employed to select asample of 1,472 subscribers from the publication's domestic circulation list.If we assume that the subscriber list had 1,472,000 names, the samplinginterval would be 1,000 (1,472,000/1,472). A number from 1 to 1,000 wasdrawn at random. Beginning with that number, every 1,000th subscriber wasselected.
A brand-new dollar bill was included with the questionnaire as an incentive to
respondents. An alert postcard was mailed one week before the survey. Asecond, follow-up, questionnaire was sent to the whole sample ten days after the initial questionnaire. There were 76 post office returns, so the neteffective mailing was 1,396. Six weeks after the first mailing, 778 completedquestionnaires were returned, yielding a response rate of 56%.
Tennis's Systematic SamplingReturns a Smash
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Stratified Sampling
Proportionate stratified samplingnumber selected from each strata is proportional tonumber in total population
Disproportionate stratified samplingsample size in each group is proportional to therespective strata size
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Random samplefrom each stratum
Not necessary todraw the samenumber from eachstratum
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Non-probability Sampling
Judgemental sampling judgement used to identify representative samples
Snowball samplingform of judgement sampling
appropriate for small specialised population
each respondent is asked to identify others in group
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Non-probability SamplingConvenience sampling
respondents selected conveniently
inexpensive
quick
Quota samplingminimum number from each specified subgroup in thepopulation
often based on demographics
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Sample Sizes Used in Marketing Research Studies
Type of Study Minimum Size Typical Range
Problem identification research(e.g. market potential)
500 1,000-2,500
Problem-solving research (e.g.
pricing)
200 300-500
Product tests 200 300-500
Test marketing studies 200 300-500
TV, radio, or print advertising (percommercial or ad tested)
150 200-300
Test-market audits 10 stores 10-20 stores
Focus groups 2 groups 4-12 groups
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Qualitative factors: Determining the Sample Size
Importance of the decision
Number of variables
Nature of the analysis
Sample size used in similar studies
Incidence rates
Completion rates
Resource constraints
Anticipated refusals
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Sampling and non sampling errors
Error can occur from the sampling processsampling error
and from other issues that are not associated with thesampling for the survey
non sampling errors.
response and non-response errors. non-sampling error largely occurs during the process of collecting the data.
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Non-response Problems
Due torefusal to respond
inability to respond
not at home/inaccessibleReduced by
improving the researchdesign
call-backs
Every effort should be madeto reduce these kinds of errorsas they are the easiest tominimise
The researcher and theinterviewer need to beprofessional and familiar withthe processes
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A sample is almost always more desirable than a censusbecause:
Cost - populations are often very large and taking acensus is expensive
Time - because of the numbers in a population it can takea very long time to access each sample unit
Analysis - typically researchers and research firms cannotprocess the huge amounts of data generated by a census
Non-sampling Error - Census can have very high non-
sampling errors when compared to sampling error, thusaccuracy can be better in a sample
Secrecy - often marketing research is done for competitiveadvantage
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Summarydistinguish between a census and a sample
describe the steps in the sampling process
define the appropriate target population for a given researchproblem
identify and evaluate alternative sampling frames for atarget population
differentiate between probability and non-probabilitysampling techniques
assess non-response problems in sampling
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For you to answer
True or false.If a researcher wanted a sample of 100
people based on a sampling frame of 10,000 people whoare subscribers to a magazine, they should sample every200th person on the list.
Sampling error is:
a) The difference between the population parameter and thesample parameter as a result of sampling
b) Error that occurs when the researcher incorrectly definesthe constructs of the study
c) Error that occurs as a result of incorrectly defining theresearch objectives
d) All of the above
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And more Which of the following best describes the sample for a shoppingmall intercept:
a) A random sampleb) A probability samplec) A non probability sample
d) None of the above
Which of the following sets of sampling techniques contains onlyprobability sampling techniques?
a) systematic sampling, quota sampling, snowball samplingb) cluster sampling, quota sampling, snowball samplingc) proportionate stratified sampling, quota sampling, snowball
sampling
d) None of the above