Sampling Issues

1
Poor
Good
Yes
No
Yes
Yes
No
No
FOCUS GROUPS
OBSERVATIONAL RESEARCH
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)
Sample
Population
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
Examples of Clusters
Population Element Possible Clusters
University seniors Universities Manufacturing
firms Counties Metropolitan Statistical
Areas Localities Plants
11
Examples of Clusters
Population Element Possible Clusters
Airline travelers Airports Planes Sports
fans Football stadiums Basketball
arenas Baseball parks
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 gt 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