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Sampling Designs

Sampling Designs. Continued…. Objectives:. To develop the basic properties of collecting an unbiased sample. To learn to recognize flaws in biased sampling. population is divided into homogeneous groups called strata SRS’s are pulled from each strata. Stratified random sample. Advantages

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Sampling Designs

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  1. Sampling Designs Continued…

  2. Objectives: • To develop the basic properties of collecting an unbiased sample. • To learn to recognize flaws in biased sampling.

  3. population is divided into homogeneous groups called strata SRS’s are pulled from each strata Stratified random sample

  4. Advantages More precise unbiased estimator than SRS Less variability Cost reduced if strata already exists Disadvantages Difficult to do if you must divide stratum Formulas for SD & confidence intervals are more complicated Need sampling frame Stratified

  5. based upon location randomly pick a location & sample all there Cluster Sample

  6. Advantages Unbiased Cost is reduced Sampling frame may not be available (not needed) Disadvantages Clusters may not be representative of population Formulas are complicated Cluster Samples

  7. select successively smaller groups within the population in stages SRS used at each stage Multistage sample

  8. Identify the sampling design The Educational Testing Service (ETS) needed a sample of colleges. ETS first divided all colleges into groups of similar types (small public, small private, etc.) Then they randomly selected 3 colleges from each group. Stratified random sample

  9. Identify the sampling design A county commissioner wants to survey people in her district to determine their opinions on a particular law up for adoption. She decides to randomly select blocks in her district and then survey all who live on those blocks. Cluster sampling

  10. some groups of population are left out of the sampling process Undercoverage

  11. occurs when an individual chosen for the sample can’t be contacted or refuses to cooperate telephone surveys 70% nonresponse Nonresponse

  12. occurs when the behavior of respondent or interviewer causes bias in the sample wrong answers Response bias

  13. wording can influence the answers that are given connotation of words use of “big” words or technical words Wording of the Questions

  14. Source of Bias? Before the presidential election of 1936, FDR against Republican ALF Landon, the magazine Literary Digest predicting Landon winning the election in a 3-to-2 victory. A survey of 10 million people. George Gallup surveyed only 50,000 people and predicted that Roosevelt would win. The Digest’s survey came from magazine subscribers, car owners, telephone directories, etc. Undercoverage

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