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Chapter 10 Sampling: Theories, Designs and Plans

1. Distinguish between probability and nonprobability sampling. 2. Explain the advantages and disadvantages of probability sampling. 3. Compare the advantages and disadvantages of nonprobability sampling. 4. Describe the factors that influence the appropriate sample design. 5. Understand the steps in developing a sampling plan. Learning Objectives 10-3

Sampling... involves selecting a relatively small number of elements (sample) from a larger defined group (population) and expecting the information gathered from the small group will enable judgments about the larger group.... involves selecting a relatively small number of elements (sample) from a larger defined group (population) and expecting the information gathered from the small group will enable judgments about the larger group. 10-4

Census... primary data is collected from every member of the target population. 10-5

Population Elements (people, products, etc.) Defined target population Sampling units (available elements) Sampling frame Sampling Terms 10-6

Types of Error SamplingSamplingNonsamplingNonsampling 10-7

... any type of bias that results from mistakes in either the selection process for prospective sampling units or in determining the sample size. Sampling Error Information on Sampling Error, etc. http://www.statcan.ca/english/edu/power/ch6/sampling/sampling.htm#samplesize 10-8

Nonsampling Error... bias that occurs in a research study regardless of whether a sample or census is used; e.g., bias caused by measurement errors, response errors, coding errors, etc. 10-9

Sampling Methods ProbabilitysamplingProbabilitysampling NonprobabilitysamplingNonprobabilitysampling 10-10

Probability Simple random sampling Simple random sampling Systematic random sampling Systematic random sampling Stratified random sampling Stratified random sampling Cluster sampling Cluster sampling Nonprobability Convenience sampling Convenience sampling Judgment sampling Judgment sampling Quota sampling Quota sampling Snowball sampling Snowball sampling Types of Sampling Methods 10-11

Cluster Sampling Simple Random Sampling Probability Sampling Systematic Random Sampling Stratified Random Sampling 10-12

Snowball Sampling Convenience Sampling Nonprobability Sampling Judgment Sampling Quota Sampling 10-13

Summary of Comparative Differences between Probability and Nonprobability Sampling Methods 10-14

Simple Random Sampling... a method of probability sampling in which every unit has a known and equal chance of being selected. 10-15

Systematic Random Sampling... a method of probability sampling in which the defined target population is ordered and the sample is selected according to position using a skip interval. 10-16

Steps in Drawing a Systematic Random Sample 1. Obtain a list of units that contains an acceptable frame of the target population. 2. Determine the number of units in the list and the desired sample size. 3. Compute the skip interval. 4. Determine a random start point. 5. Beginning at the start point, select the units by choosing each unit that corresponds to the skip interval. 10-17

... a method of probability sampling in which the population is divided into different subgroups and samples are selected from each.... a method of probability sampling in which the population is divided into different subgroups and samples are selected from each. Stratified Random Sampling 10-18

Selecting a Stratified Random Sample – Three Basic Steps – (3) Combine samples from each stratum into a single sample. (2) Select random samples from each stratum. (1) Divide population into homogeneous subgroups. 10-19

Stratified Sampling Two Methods of Deriving Samples from Strata ProportionateProportionateDisproportionateDisproportionate 10-20

Cluster Sampling... a method of probability sampling where the sampling units are selected in groups rather than individually.... a method of probability sampling where the sampling units are selected in groups rather than individually. 10-21

Steps in Drawing a Two-Step Cluster Sample 10-22

Nonprobability Sampling Methods Convenience sampling... relies upon convenience and access. Convenience sampling... relies upon convenience and access. Judgment sampling... relies upon belief that participants fit characteristics. Judgment sampling... relies upon belief that participants fit characteristics. Quota sampling... emphasizes representation of specific characteristics. Snowball sampling... relies upon respondent referrals of others with like characteristics. Snowball sampling... relies upon respondent referrals of others with like characteristics. 10-23

Factors to Consider in Sample Design Research objectives Degree of accuracy ResourcesResources Time frame Knowledge about target population Knowledge about target population Research scope Statistical analysis requirements 10-24

Critical Factors in Selecting the Appropriate Sampling Design 10-25

Factors Affecting Sample Size Variability of the population characteristic under investigation. Variability of the population characteristic under investigation. Level of confidence desired in the estimate, most often 95% (within 2 SD). Level of confidence desired in the estimate, most often 95% (within 2 SD). Degree of precision desired in estimating the population characteristic, e.g. sampling error = +/- 2. Degree of precision desired in estimating the population characteristic, e.g. sampling error = +/- 2. Other... Other... oFormulas oRules of Thumb http://www.surveysystem.com/sscalc.htm Sample Size Calculator 10-26

... the process used to select sampling units from the population. Survey Sampling is a limited Service supplier firm that specializes in helping firms with their sampling plans. www.surveysampling.com 10-27

1. Define the Target Population 2. Select the Data Collection Method 3. Identify the Sampling Frame(s) Needed 4. Identify the Appropriate Sampling Method 5. Determine Sample Sizes and Contact Rates 6. Create Plan for Selecting Sampling Units 7. Execute the Plan Steps in Developing a Sampling Plan 10-28

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