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Chapter 13 Sampling Designs

Chapter 13 Sampling Designs. Basic Sampling Concepts in Quantitative Studies. Population The aggregate of cases in which a researcher is interested Sampling Selection of a portion of the population (a sample) to represent the entire population.

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Chapter 13 Sampling Designs

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  1. Chapter 13Sampling Designs

  2. Basic Sampling Concepts in Quantitative Studies Population The aggregate of cases in which a researcher is interested Sampling Selection of a portion of the population (a sample) to represent the entire population

  3. Basic Sampling Concepts in Quantitative Studies (cont’d) Probability sampling Involves random selection of elements Non-probability sampling Does not involve selection of elements at random

  4. Basic Sampling Concepts in Quantitative Studies (cont’d) Representative sample A sample whose key characteristics closely approximate those of the population Sampling bias The systematic over- or under- representation of segments of the population on key variables Sampling error Differences between population and sample values

  5. Nonprobability Sampling • Convenience (accidental) sampling • Snowball (network) sampling • Quota sampling • Purposive sampling

  6. Table 13.1 Textbook Example: Numbers (Percentages) of Students in Strata of a Population and Convenience & Quota Sample

  7. Table 13-2 Students Willing to Work on AIDS Unit in Population and Nonprobability Samples

  8. Probability Sampling • Simple random sampling • Stratified random sampling • Cluster (multistage) sampling • Systematic sampling

  9. Table 13.3 Sampling Frame for Simple Random Sampling Example

  10. Table 13.4 Comparison of Population and Sample Values and Averages

  11. Table 13-5 Three Populations of Different Homogeneity

  12. Considerations That Affect Sample Size in Quantitative Studies • Homogeneity of the population • Effect size (strength of relationships) • Attrition (loss of subjects) • Interest in subgroup analyses • Sensitivity of the measures

  13. Sampling Steps in Quantitative Studies • Identify the population • Specify the eligibility criteria • Specify the sampling plan (sampling method and sample size) • Recruit the sample

  14. Factors Affecting Recruitment Success • Method of recruitment (face-to-face, by mail, etc.) • Courtesy • Persistence • Incentives (gifts, monetary payments) • Research benefits

  15. Factors Affecting Recruitment Success (cont’d) • Participants’ access to study results • Convenience • Endorsements • Assurances of confidentiality

  16. Methods of Sampling in Qualitative Research • Convenience (volunteer) sampling • Snowball sampling • Theoretical sampling • Purposive sampling

  17. Types of Purposive Sampling in Qualitative Research (Examples) • Maximum variation sampling • Homogenous sampling • Extreme (deviant) case sampling • Intensity sampling • Sampling confirming/disconfirming cases

  18. Sample Size in Qualitative Research • No explicit, formal criteria • Sample size determined by informational needs • Decisions to stop sampling guided by data saturation • Data quality can affect sample size

  19. Sampling in the Three Main Qualitative Traditions • Ethnography • Mingling with many members of the culture • Informal conversations with 25 to 50 informants • Multiple interviews with smaller number of key informants

  20. Sampling in the Three Main Qualitative Traditions (cont’d) • Phenomenology • Relies on very small samples (often 10 or fewer) • Participants must have experienced phenomenon of interest

  21. Sampling in the Three Main Qualitative Traditions (cont’d) • Grounded theory • Typically involves samples of 20 to 30 people • Selection of participants who can best contribute to emerging theory (usually theoretical sampling)

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