SAMPLING

1. SAMPLING Chapter 12 Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

2. Learning Outcomes • Identify the purpose of sampling. • Define population, sample, and sampling. • Compare and contrast a population and a sample. • Discuss the eligibility criteria for sample selection. • Define nonprobability and probability sampling. • Identify the types of nonprobability and probability sampling strategies. • Compare the advantages and disadvantages of specific nonprobability and probability sampling strategies. Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

3. Learning Outcomes (cont’d) • Discuss the contribution of nonprobability and probability sampling strategies to strength of evidence provided by study findings. • Discuss the factors that influence determination of sample size. • Discuss the procedure for drawing a sample. • Identify the criteria for critiquing a sampling plan. • Use the critiquing criteria to evaluate the “Sample” section of a research report. Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

4. Sampling Concepts • Population—a well-defined set (ofwhat?) that has certain properties • People • Animals • Objects • Events Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

5. Identifying Population Descriptors • Specify inclusion (eligibility) criteria • Specify exclusion (delimitations) criteria . . . leads to sample selection Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

6. Gender Age Marital status Religion Ethnicity Education Health status Diagnosis Co-morbidities Population Descriptor Examples Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

7. Think about the concept of inclusion or eligibility criteria applied to a research study where the subjects are patients. For example, participants in a study investigating the pain experience of children with leukemia during the first year after diagnosis (Van Cleve et al., 2004) had to meet the following inclusion (eligibility) criteria: Age – 4 to 17 years Diagnosis – acute lymphocytic leukemia within 1 month of diagnosis Health status – no other existing chronic illness associated with pain, a fulminating disease, or a known cognitive disability Status – ability to cope with the burden of research tasks as determined by the primary nurse Language – English or French speaking Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

8. Target populationvs.accessible population Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

9. Samples and Sampling • Sampling is a process of selecting a portion or subset of the designated population to represent the entire population. • A sample is a set of elements that make up the population. • An element is the most basic unit about which information is collected. Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

10. Sample Elements • People • Places • Objects Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

11. Representativeness • A representative sample is one whose key characteristics closely approximate those of the population. Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

12. Types of Sampling Strategies • Non-probability • Probability Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

13. Non-probability Sampling • Convenience sampling • Quota sampling (a proportional share) • Purposive sampling • Matching Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

14. Purposive Sampling • Effective pretesting of newly developed instruments with a purposive sample of divergent types of people • Validation of a scale or test with a known-groups technique • Collection of exploratory data in relation to an unusual or highly specific population, particularly when the total target population remains an unknown to the researcher(s) Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

15. Purposive Sampling (cont’d) • Collection of descriptive data (e.g., as in qualitative studies) that seek to describe the lived experience of a particular phenomenon (e.g., postpartum depression, caring, hope, or surviving childhood sexual abuse) • Focus of the study population relates to a specific diagnosis (e.g. type I diabetes, multiple sclerosis) or condition (e.g., legal blindness, terminal illness) or demographic characteristic (e.g., same-sex twins) Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

16. Probability Sampling • Simple random • Stratified random • Multi-stage (cluster) • Systematic • Matching Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

17. Factors Influencing Sample Size • Type of design used • Type of sampling procedure used • Type of formula used for estimating optimum sample size • Degree of precision required • Heterogeneity of the attributes under investigation • Relative frequency that the phenomenon of interest occurs in the population (i.e., common vs. rare health problem) • Projected cost of using a particular sampling strategy Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12

18. Critical Thinking Decision Path: Assessing the Relationship between the Type of Sampling Strategy and the Appropriate Generalizability Padre Fred / MLT/CSHS: Research Methodology; 2011 – 12