learning objectives copyright © 2002 south-western/thomson learning basic sampling issues chapter...
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Learning ObjectivesLearning Objectives
Copyright © 2002 South-Western/Thomson Learning
Basic Sampling Issues
CHAPTER
twelve
CHAPTER
twelve
Learning ObjectivesLearning ObjectivesLearning Objectives
1. To understand the concept of sampling.
2. To learn the steps in developing a sampling plan.
3. To understand the concepts of sampling error and nonsampling error.
4. To distinguish between probability samples, and nonprobability samples.
5. To understand sampling implications of surveying over the Internet.
Learning ObjectivesLearning Objectives
Sampling Defined:The process of obtaining information from a subset of a larger group.
A market researcher takes the results from the sample to make estimates of the larger group.
Sampling a small percentage of a population can result in very accurate estimates.
To understand the concept of sampling.
The Concept of Sampling
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Population or Universe
The total group of people from whom we need to obtain information.
Define the target market for the product or service in question.
Sample versus Census
Census: Data about every member of the population.
Sample: A subset of the population
Definition Of Important Terms
To understand the concept of sampling.
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Step1.Define the
Population of Interest
Step 2. ChooseData Collection
Method
Step 3.Choose
Sampling Frame
(4) Select a
Sampling Method
Step 5. Determine
Sample Size
Step 6. Develop Operational Plan
Step 7.Execute
Operational Plan
Figure 12.1 Steps in Developing a Sample Plan
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Step One: Defining the Population of Interest
Specifying the characteristics from whom information is needed.
Define the characteristics of those that should be excluded.
Step Two: Choose Data Collection Method
Impacts for the sampling process.
Step Three: Choosing Sampling Frame
A list of elements or members from which we select units to be sampled.
To learn the steps in developing a sample plan.
Steps In Developing A Sampling Plan
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Step Four: Select a Sampling Method
The selection will depend on:
• The objectives of the study
• The financial resources available
• Time limitations
• The nature of the problem
Probability Samples
A known, nonzero probability of selection
To learn the steps in developing a sample plan.
Steps In Developing A Sampling Plan
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Nonprobability Samples
Elements selected in a nonrandom manner.
Advantages of probability samples
• Obtain information from a representative cross-section
• Sampling error can be computed The survey results are projectable to the total population.
To understand the steps in developing a sample plan.
Steps In Developing A Sampling Plan
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Disadvantages of probability samples
• More expansive than nonprobabiity samples of the same size.
• Probability samples take more time to design and execute.
Step Five: Determine Sample Size
• Available budget
• Rules of thumb
• Number of subgroups
To distinguish between probability samples and nonprobability samples.
Steps In Developing A Sampling Plan
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Step Six: Develop of Operational Procedures for Selecting Sample Elements
Specify whether a probability or nonprobability sample is being used
Step Seven: Execution the Sampling Plan
The final step of the operational sampling plan
Include adequate checking of specified procedures.
To distinguish between probability samples and nonprobability samples.
Steps In Developing A Sampling Plan
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Sampling methods
Probability samples
Systematic
Cluster
Stratified
Simple random
Nonprobability samples
Convenience
Judgement
Snowball
Quota
Figure 12.2 Classification of Sampling Methods
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Sampling ErrorThe error that results when the same sample is not perfectly representative of the population.
Two types of sampling error:
To understand the concepts of sampling error and nonsampling error.
Sampling And Nonsampling Errors
+- sns
+-X = X = sample mean
= true population mean
s = sampling error
ns = nonsampling error
Learning ObjectivesLearning Objectives
Sampling ErrorThe error that results when the same sample is not perfectly representative of the population.
• Administrative error: problems in the execution of the sample
• Random error: due to chance and cannot be avoided
To understand the concepts of sampling error and nonsampling error.
Sampling And Nonsampling Errors
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Simple Random SamplingThe purest form of probability sample
To understand the differences in probability and nonprobability sampling methods.
Probability of Selection = Sample Size
Population Size
Systematic SamplingUses a fixed skip interval to draw elements from a numbered population.
Skip Interval = Population Size
Sample Size
Probability Sampling Methods
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Stratified SamplesProbability samples that are distinguished by the following steps:
1. The original population is divided into two or more mutually exclusive and exhaustive subsets
2. Simple random samples of elements from the two or more subsets are chosen independently from each other.
To understand the differences in probability and nonprobability sampling methods.
Probability Sampling Methods
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Three steps involved in implementing a properly stratified sample:
1. Identify salient demographic or classification factors correlated with the behavior of interest.
2. Determine what proportions of the population fall into various sub subgroups under each stratum.
• proportional allocation
• disproportional or optimal allocation
To understand the differences in probability and nonprobability sampling methods.
Probability Sampling Methods
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3. Select separate simple random samples from each stratum.
Cluster SamplesSampling units are selected in groups.
1. The population of interest is divided into mutually exclusive and exhaustive subsets.
2. A random sample of the subsets is selected.
To understand the differences in probability and nonprobability sampling methods.
Probability Sampling Methods
Learning ObjectivesLearning ObjectivesTo understand the differences in probability and nonprobability sampling methods.
Convenience Samples
Easy to collect
Judgement Samples
Based on judgmental selection criteria
Quota Samples
Demographic characteristics in the same proportion as in the population
Snowball Samples
Additional respondents selected on referral from initial respondents.
Nonprobability Sampling Methods
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To understand sampling implications of surveying over the Internet.
Advantages of Internet sampling:
• Target respondents can complete the survey at their convenience
• Data collection is inexpensive
• The interview can be administered under software control
• The survey can be completed quickly
Sampling Over the Internet
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Don’t Post Surveys in Public Areas
• Highly biased samples
• Vested interests quickly
To understand sampling implications of surveying over the Internet.
Sampling Over the Internet
Learning ObjectivesLearning Objectives
• The Concept of Sampling
• Definition Of Important Terms
• Steps In Developing A Sampling Plan
• Sampling And Nonsampling Errors
• Probability Sampling Methods
• Nonprobability Sampling Methods
• Sampling Over the Internet
SUMMARY
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The End
Copyright © 2002 South-Western/Thomson Learning