1. Chapter 10 Sampling: Theories, Designs and Plans

2.  Sampling Terms Sampling - selecting a small number of elements (sample) from a larger defined group (population). Census - data is collected from every member of the target population. Sampling - selecting a small number of elements (sample) from a larger defined group (population). Census - data is collected from every member of the target population. 10-2

3.  Population – everyone in the universe Elements – specific units of interest in the population. (They can be people, products, stores, etc.) Defined target population – population of interest in the study Sampling units – individual elements chosen in the sample Sampling frame – list of potential population elements Other Sampling Terms 10-3

4.  Sampling Error – any type of bias that results from mistakes in either the selection process of sampling units or in determining the sample size. http://www.statcan.ca/english/edu/power/ch6/sampling/sampling.htm#samplesize Sampling Error – any type of bias that results from mistakes in either the selection process of sampling units or in determining the sample size. http://www.statcan.ca/english/edu/power/ch6/sampling/sampling.htm#samplesize Types of Errors 10-4 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.

5.  Probability  Simple random  Systematic random  Stratified random  Cluster sampling Nonprobability  Convenience sampling  Judgment sampling  Quota sampling  Snowball sampling Types of Sampling Methods 10-5

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7.  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. Steps in Drawing a Systematic Random Sample 10-7

8.  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. 10-8

9.  1.Identify target population and determine clustering factors. (people type, store type, etc). 2.Determine the number of units in each cluster. 3.Determine the number of units in each cluster are needed to represent the cluster. 4.Randomly select units within each cluster. 5.Combine cluster samples into one study sample. Different than stratified because of the increased need for knowledge of the population and cluster groups to be able to accurately complete it. Steps in Drawing a Cluster Sample 10-9

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

11.   Variability of the population characteristic under investigation.  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.  Other... oFormulas oRules of Thumb Factors Affecting Sample Size http://www.surveysystem.com/sscalc.htm Sample Size Calculator 10-11

12.  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 10-12