SAMPLING
• Census -- the entire population
– most useful is the population ("n") is small
– or the cost of making an error are high
• Sample -- contacting a portion of the
population (e.g., 10% or 25%)
– best with a very large population (n)
– easiest with a homogeneous population
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Sampling -- graphically
Population
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The sampling process
1. Determine the target population
who are the people you want information on?
age, gender, product use
2. Determine the sampling frame
how will you get the names, phone numbers or
addresses?
– existing lists, phone book, random digit dialing
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The sampling process
3. Select a sampling procedure:
A. Probability (random) sample
– equal chance of being included in the sample
– random number table, even-odd, etc.
B. Stratified
– equalizing "important" variables
• year in school, geographic area, product use, etc.
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The sampling process
C. Nonprobability sampling
– convenience sample
• people in my classes
– "snowball" sample
• friends of friends
– "quota" sampling
• 50 women, 50 men
– mall intercepts
• Park Place Mall
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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.
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Instead of this (good)…
Population
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…this (bad)…
Sample
Population
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…or this (VERY bad)…
Sample
Population
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Drawing inferences from samples
• Population estimates
– % who smoke, buy your product, etc
• 25% of sample
• what % of population?
– very dangerous with a non-representative
sample or with low response rates
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Drawing inferences from samples
• Relationships
– e.g., exposure to ads and liking for the product
– relationships (qualitatively different)
– some danger with a non-representative sample
• less problematic in experiments
– (other factors are controlled)
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Sampling messages
• To be able to draw inferences about
a sample of messages
– (e.g., "funny" ads vs. "informational" ads)
• you should also sample from the
universe of those messages.
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Instead of this (bad)….
Messages
1
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…this (good)
Messages
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The End
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