1. Need estimates of prevalence? Need to establish causality? EXPERIMENT Yes Objective answers by asking? No Yes SURVEY Good Poor Understanding of problem? Understanding of Problem? EXPLORATORY or SECONDARY RESEARCH No OBSERVATIONAL RESEARCH FOCUS GROUPS No

2. Sampling Issues Chapter 16

3. Key Terms Population: group of interest Census: study of whole population Sample: subset of population Sample frame: master list of population Sampling error: error due to sample issues Representative: mirrors the population Sample unit: object of study

4. Sampling Units Group selected for the sample Primary Sampling Units (PSU) Secondary Sampling Units Tertiary Sampling Units

5. Sampling – you want this: Population Sample

6. …not this (bad)… Population Sample

7. …or this (VERY bad)… Population Sample

8. Generalization You can only generalize to the population from which you sampled –U of L students not college students geographic, different majors, different jobs, etc. –College students not Canadian population younger, poorer, etc. –Canadians not people everywhere less traditional, more affluent, etc.

9. Probability Samples Known likelihood of selection Simple random –Blind draw –Random numbers Systematic –Random first, skip interval Stratified –Sample from subgroups Cluster –1 step –2 step

10. Population ElementPossible Clusters University seniorsUniversities Manufacturing firmsCounties Metropolitan Statistical Areas Localities Plants Examples of Clusters

11. Population ElementPossible Clusters Airline travelersAirports Planes Sports fansFootball stadiums Basketball arenas Baseball parks Examples of Clusters

12. Nonprobability Samples Unknown likelihood of selection Convenience Judgement Referral Quota

13. Sample Plan Define population Attain sample frame Design sample plan Draw sample Assess sample (Resample if necessary)

14. Sample Size Issues Census is the only perfect sample –all probability samples have error Larger samples have less sampling error Accuracy independent of population size Size depends on accuracy vs. resources trade-off

15. Sample Size Heuristics If n > 500, increase in size not much help Increase confidence from 95% to 99% increases sample by 73%. Sample size calculator: –http://www.surveysystem.com/sscalc.htm More on this next chapter.

16. The End