Chapter 6

1. Chapter 6 Selection of Research Participants: Sampling Procedures Conducting & Reading Research Baumgartner et al

2. Population and Sample • entire group or aggregate of people or elements having one or more common characteristics • small subgroup of a population of interest thought to be representative of the larger population Conducting & Reading Research Baumgartner et al

3. Steps in sampling process • Identify the target population • Identify the accessible population • Determine the desired sample size • Select the specific sampling technique • Implement the sampling plan • Goal is to have a sample Conducting & Reading Research Baumgartner et al

4. Random processes • Random : selecting a sample that is representative of the population • Random : establish group equivalence by randomly assigning research participants to treatment conditions or comparison groups Conducting & Reading Research Baumgartner et al

5. Sample Selection Methods • Probability sampling: random processes are used to select members of the sample • Nonprobability sampling: random processes not used Conducting & Reading Research Baumgartner et al

6. Probability Sampling • Simple random sampling – each item has equal chance of being selected • Stratified random sampling-divide into subgroups and select # from each group • Systematic sampling-use system, such as every kth element • Cluster sampling-usually use naturally occurring group Conducting & Reading Research Baumgartner et al

7. Nonprobability Sampling • Purposive sampling (selected sampling) – • Convenience sampling – Conducting & Reading Research Baumgartner et al

8. Sample Size • How many subjects do you need? • Important for statistical applications • Must plan to nonresponse of some subjects • Random sampling is best if can be achieved • Must worry about • variation due to chance that exists between a population parameter and a simple statistic Conducting & Reading Research Baumgartner et al

9. Considerations for sample size • Sampling error inversely related to sample size. • Descriptive & correlational research needs larger samples. • Sample size should increase as variability within a population increases • If dividing sample into smaller groups, make initial sample big enough • Practical factors such as subject availability and costs must be considered • Power to detect a meaningful effect is related to sample size Conducting & Reading Research Baumgartner et al

10. Sample size calculators • calculators.stat.ucla.edu/sampsiz.php • www.surveysystem.com/sscale/htm Conducting & Reading Research Baumgartner et al