Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter 13 Developing a Sampling Plan

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Basic Sampling Concepts in Quantitative Studies (cont’d) Probability sampling Involves random selection of elements Nonprobability sampling Does not involve selection of elements at random

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Nonprobability Sampling Convenience (accidental) sampling Snowball (network) sampling Quota sampling Purposive sampling

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Numbers and Percentages of Students in Strata of a Population, Convenience Sample, and Quota Sample

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Students Willing to Work on AIDS Unit: Population, Convenience Sample, and Quota Sample

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Comparison of Population and Sample Values and Averages: Nursing Home Aspirin Consumption Example

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Three Populations of Different Homogeneity

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Probability Sampling Simple random sampling Stratified random sampling Cluster (multistage) sampling Systematic sampling

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Sampling Steps in Quantitative Studies Identify the population Specify the eligibility criteria Specify the sampling plan (sampling method and sample size) Recruit the sample

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Factors Affecting Recruitment Success Method of recruitment (face-to-face, by mail, etc.) Courtesy Persistence Incentives (gifts, monetary payments) Research benefits

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Factors Affecting Recruitment Success (cont’d) Participants’ access to study results Convenience Endorsements Assurances of confidentiality

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Methods of Sampling in Qualitative Research Convenience (volunteer) sampling Snowball sampling Theoretical sampling Purposive sampling

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Types of Purposive Sampling in Qualitative Research (Examples) Maximum variation sampling Homogenous sampling Extreme (deviant) case sampling Intensity sampling Sampling confirming/disconfirming cases

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins 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)

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Levels of Measurement Nominal measurement Involves assigning numbers to classify characteristics into categories Ordinal measurement Involves sorting objects based on their relative standing on an attribute

Copyright © 2008 Wolters Kluwer Health | Lippincott Williams & Wilkins Levels of Measurement (cont.) Interval measurement Occurs when objects are rank-ordered on a scale that has equal distances between points on the scale Ratio measurement Occurs when there are equal distances between score units and there is a rational, meaningful zero