1. Research Sampling Procedures, Methods, & Issues
Rick Carlton
Fall 1999

2. Nonprobability Sampling
Provides a statistical basis for verifying that sample is representative of population
Every member of the population has a known, nonzero probability of being included in the sample
Based on random selection techniques
Nonprobability Sampling
Sample chosen based on judgement regarding population characteristics and survey needs
Limitations on generalizing findings

3. Probability Sampling Simple Random Sampling Stratified Random Sampling
Systematic Sampling
Cluster Sampling

4. Probability Sampling Simple Random Sampling
Sampling frame must include all or nearly all members of the population
All members have an equal chance of selection
No sensitivity to subgroup differences

5. Probability Sampling Stratified Random Sampling
Population is divided into subgroups to measure selected factors
Subgroup identification can be through literature or expert opinion

6. Probability Sampling Systemic Sampling
For populations without naturally occuring repetition, choosing systematically (I.e., every other name) can assure randomness in selection

7. Probability Sampling Cluster Sampling
Uses naturally occuring units (i.e., one entire school)
Multistage cluster sampling uses random sampling from naturally occuring clusters
Increasing number of clusters allows decreasing sample size within each cluster
Useful in permitting use of more clusters or more samples, depending on which is easier to access

8. Nonprobability Sampling Rationale.
Helps with hard to identify groups where cooperation and completion of interview or survey may be difficult
Assists when sampling groups where some members may be incapable of cooperating or where ethical considerations may limit some data collection techniques
Provides pilot study data for making decisions about feasibility of a study

9. Nonprobability Sampling
Convenience Sampling
Snowball Sampling
Quota Sampling
Focus Groups

10. Nonprobability Sampling
Convenience Sampling
Ready and available sample
Very low (or nonexistant) generalizability

11. Nonprobability Sampling
Snowball Sampling
Relies on previously identified members of a group to identify other members of the population
Useful when population listings are not available

12. Nonprobability Sampling
Quota Sampling
Use subgroups and sample proportionately
Requires current records to obtain accurate sample

13. Nonprobability Sampling
Focus Groups
Provides in-depth portraits
Unperceived uniqueness can skew results dramatically

14. Purposeful Sampling In Qualitative Research
Deviant Case
Intensity
Typical Case
Maximum Variation
Stratified Purposeful
Homogeneous
Critical Case
Snowball or Chain
Criterion
Theory-Based
Confirming and Disconfirming
Purposeful Random
Politically Important Cases
Convenience