Understanding
of problem?
Problem?
Poor
EXPLORATORY or
SECONDARY
RESEARCH
Good
Need to establish
causality?
Yes
EXPERIMENT
No
Objective
answers by
asking?
No
OBSERVATIONAL
RESEARCH
Yes
Need estimates
of prevalence?
No
FOCUS GROUPS
Yes
SURVEY
Sampling Issues
Chapter 16
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
Sampling Units
•
•
•
•
Group selected for the sample
Primary Sampling Units (PSU)
Secondary Sampling Units
Tertiary Sampling Units
Sampling – you want this:
Population
Sample
…not this (bad)…
Sample
Population
…or this (VERY bad)…
Sample
Population
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.
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
Examples of Clusters
Population Element
Possible Clusters
University seniors
Manufacturing firms
Universities
Counties
Metropolitan Statistical Areas
Localities
Plants
Examples of Clusters
Population Element
Possible Clusters
Airline travelers
Airports
Planes
Sports fans
Football stadiums
Basketball arenas
Baseball parks
Nonprobability Samples
Unknown likelihood of selection
•
•
•
•
Convenience
Judgement
Referral
Quota
Sample Plan
•
•
•
•
•
•
Define population
Attain sample frame
Design sample plan
Draw sample
Assess sample
(Resample if necessary)
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
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.
The End