Sampling

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CHAPTER   Ten

Learning Objectives

Basic Sampling  Issues

Copyright © 2004 John Wiley & Sons, Inc.

Learning Objectives Learning 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 understand the differences between  probability samples, and nonprobability  samples. 5. To understand sampling implications of  surveying over the Internet.

Learning Objectives

The Concept of Sampling

To understand the  concept of sampling.

Sampling Defined: 1. The process of obtaining information from a subset of  a larger group. 2. A market researcher takes the results from the  sample to make estimates of the larger group. 3. Sampling a small percentage of a population can  result in very accurate estimates. 

Definition Of Important  Terms

Learning Objectives To understand the  concept of sampling.

Population or Universe 1. The population or population of interest is the total  group of people from whom information is needed. 2. Defining the population of interest is the first step in  the sampling process 3. Requires good logic and judgment 4. Based on the characteristics of current or target  customer Sample versus Census  Census:  Data about every member of the population. Sample:  A subset of the population

Figure 10.1

Learning Objectives Steps in Developing a Sample Plan

Step 7. Execute  Operational Plan

Step 6. Develop  Operational Plan

Step 5.  Determine  Sample Size

Step 2. Choose Data Collection  Method Step1. Define the  Population of  Interest

Step 3. Choose  Sampling Frame

(4)   Select a  Sampling Method

Steps In Developing A  Sampling Plan

Learning Objectives To learn the steps in  developing a sample plan.

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.

Steps In Developing A  Sampling Plan

Learning Objectives To learn the steps in  developing a sample plan.

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

Steps In Developing A  Sampling Plan

Learning Objectives To understand the steps in  developing a sample plan.

Nonprobability Samples Elements selected in a nonrandom manner.  1. Nonrandomness—selected on the basis of  convenience 2. Purposeful nonrandomness—systematically  excludes or overrepresents certain subsets of the  population

Steps In Developing A  Sampling Plan

Learning Objectives To understand the steps in  developing a sample plan.

Advantages Of Probability Samples 1. Information from a representative cross­section 2. Sampling error can be computed  3. Results are projectable to the total population.

           Disadvantages Of Probability Samples

1. More expansive than nonprobabiity samples 2. Take more time to design and execute.

Steps In Developing A  Sampling Plan

Learning Objectives To understand the steps in  developing a sample plan.

Disadvantages of Nonprobability Samples 1. Sampling error cannot be computed 2. Representativeness of the sample is not known 3. Results cannot be projected to the population.

           Advantages of Nonprobability Samples 1. Cost less than probability 2. Can be conducted more quickly

3. Produces samples that are reasonably  representative

Figure 10.2

Learning Objectives Classification of Sampling Methods

Sampling  methods

Probability  samples

Systemati c

Stratified

Cluster

Simple  random

Nonprobabilit y samples

Convenienc e

Judgement

Snowball

Quota

Steps In Developing A  Sampling Plan

Learning Objectives To distinguish between probability  samples and nonprobability samples.

Step Five: Determine Sample Size • Discussed more in depth in Chapter 11 • Acceptable Error • Levels of Confidence

Steps In Developing A  Sampling Plan

Learning Objectives To distinguish between probability  samples and nonprobability samples.

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.

Sampling And  Nonsampling Errors

Learning Objectives To understand the concepts of  sampling error and nonsampling  error.

Sampling Error The error that results when the same sample is not perfectly  representative of the population. Two types of sampling error:

X =  µ +­ εs +­

εns

X = sample mean 

µ = true population mean εs = sampling error εns = nonsampling error

Sampling And  Nonsampling Errors

Learning Objectives To understand the concepts of  sampling error and nonsampling  error.

Sampling Error The 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 Measurement or Nonsampling Error Includes everything other than sampling error that can  cause inaccuracy and bias

Probability Sampling  Methods

Learning Objectives To understand the differences in   probability and nonprobability  sampling methods.

Simple Random Sampling The purest form of probability sample Sample Size

Probability of Selection =  Population Size Systematic Sampling Uses a fixed skip interval to draw elements from a  numbered population. Skip Interval = 

Population Size Sample Size

Probability Sampling  Methods

Learning Objectives To understand the differences in  probability and nonprobability  sampling methods.

Stratified Samples Probability 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.

Probability Sampling  Methods

Learning Objectives To understand the differences in   probability and nonprobability  sampling methods.

Three steps: 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

3. Select separate simple random samples from each  stratum

Probability Sampling  Methods

Learning Objectives To understand the differences in   probability and nonprobability  sampling methods.

Cluster Samples Sampling units are selected in groups. 1.  The population of interest is divided into mutually  exclusive and exhaustive subsets. 1. A random sample of the subsets is selected. • One­stage cluster—all elements in subset  selected • Two­stage cluster—elements selected in some  probabilistic manner from the selected subsets

Nonprobability Sampling  Methods

Learning Objectives To 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.

     Internet Sampling

Learning Objectives 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

      Internet Sampling 

Learning Objectives To understand sampling implications  of surveying over the Internet.

Disadvantages of Internet Interviewing 3.

users of the internet are not representative of the     general population 4. no comprehensive and reliable source of email  addresses exists

SUMMARY

Learning 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

Learning Objectives

The  End

Copyright © 2004 John Wiley & Sons, Inc.

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