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7: Sampling Theory and Methods

7: Sampling Theory and Methods. Sampling. Sampling is the process of selecting a small number of elements from a larger defined target population of elements such that the information gathered from the small group will allow judgments to be made about the larger population. Census.

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7: Sampling Theory and Methods

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  1. 7: Sampling Theory and Methods

  2. Sampling Sampling is the process of selecting a small number of elements from a larger defined target population of elements such that the information gathered from the small group will allow judgments to be made about the larger population.

  3. Census

  4. Basics of Sampling Theory Population Element Defined target population Sampling unit Sampling frame

  5. Sampling Error Sampling error is any type of bias that is attributable to mistakes in either drawing a sample or determining the sample size.

  6. Nonsampling Error Nonsampling error is bias that occurs in a research study regardless of whether a sample or census is used, such as bias caused by measurement error, response errors, non-response error or coding errors.

  7. Sampling Methods Probability sampling Nonprobability sampling

  8. Probability Simple random sampling Systematic random sampling Stratified random sampling Cluster sampling Nonprobability Convenience sampling Judgment sampling Quota sampling Snowball sampling Types of Sampling Methods

  9. Simple Random Sampling Simple random sampling is a method of probability sampling in which every unit has an equal nonzero chance of being selected.

  10. Systematic Random Sampling Systematic random sampling is 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.

  11. 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: SI = [# sampling units available/desired sample size] • 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

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

  13. Steps in Drawing a Stratified Random Sample • 1: Divide the target population into homogeneous subgroups or strata • 2: Draw random samples from each stratum • 3: Combine the samples from each stratum into a single sample of the target population

  14. Cluster Sampling Cluster sampling is a method of probability sampling where the sampling units are selected in groups rather than individually. The chosen groups are sampled in their entirety.

  15. Nonprobability Sampling Methods Convenience sampling relies upon convenience and access 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

  16. Factors to Consider in Sample Design Research objectives Degree of accuracy Resources Time frame Knowledge of target population Research scope Statistical analysis needs

  17. Factors Affecting Sample Size for Probability Designs • Variability of the population characteristic under investigation • Level of confidence desired in the estimate • Degree of precision desired in estimating the population characteristic

  18. Probability Sampling and Sample Sizes • When estimating a population mean n = (Z2B,CL)(σ2/e2) • When estimating a population proportion n = (Z2B,CL)([P x Q]/e2)

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

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