Transcript Representative Sampling

Representative Sampling
Presented at the AWDS Task Force’s
Marketing Workshop
Big Sky, Montana
Friday, September 20, 2002
Len Singel, AWDS Coordinator
Overview
 Definitions
 Reducing Error
 Sample Creation
 Examples
Definitions
 Population – consists of all the units
(individuals, households, organizations) to
which you desire to generalize survey results
 Sample Frame – list from which your sample
is drawn
Definitions
 Sample – the set of respondents selected
from a larger population for the purpose of a
survey
 Completed Sample – consists of all units that
return completed questionnaires
(instruments)
Definitions
 Coverage Error – results from every unit in
the Survey Population not having a known,
non-zero chance of being included in the
sample
 Sampling Error – the result of collecting data
from a subset, rather than all of the members
of the sampling frame
Avoiding Coverage Error
How to Reduce Coverage Error?
 Make certain your list contains everyone in
the Survey Population

Unique, non-repeating Sampling Units
 Update & Properly maintain your list
 Have your list contain other information that
can be used to improve your survey
How Large Should Your
Sample be?
 Sample does not have to be arbitrarily
selected
 ALL THAT IS REQUIRED:

Knowing about your study population
AND

Knowing your survey objectives
How Large Should Your
Sample Be?
 Sample Size Depends On:

How much sampling error can be tolerated

Population size

Variation of Population (50/50 or 80/20)

Amount of Confidence
Does Size Matter
Small Populations
Population Size
100
500
1,000
5,000
Sample Size for 95 Percent Confidence
+3%
+5%
Sampling Error
Sampling Error
50/50
80/20
50/50
80/20
Split
Split
Split
Split
92
341
516
87
289
406
80
217
278
71
165
198
880
601
357
234
Larger Populations
Population Size
10,000
50,000
100,000
1,000,000
Sample Size for 95 Percent Confidence
+3%
+5%
Sampling Error
Sampling Error
50/50
80/20
50/50
80/20
Split
Split
Split
Split
964
1,045
639
674
370
381
240
245
1,056
1,066
678
682
383
384
245
246
3 Easy Steps to Sampling
1) Identify your Target Population
2) Put together a Population List
3) Select the Sample
Hey, Wait a Minute…..
I DON’T KNOW HOW TO
SELECT THE SAMPLE!!!
Sampling Methods
 Probability


Simple Random Sampling (SRS)
Systematic Sampling
 Nonprobability (Purposeful)
Probability Sampling Method
 Simple Random Sampling (SRS)


Each member of the target population has an equal chance of
being selected
All elements are selected randomly – CAN BE
CUMBERSOME!!!
 Systematic Sampling


Members of the target population have an equal chance of being
selected
Only the first element is selected randomly; rest selected
systematically (every 5th, 10th, …)
Purposeful Sampling Method
 Depends on subjective judgment

Selected because it is convenient or typical
 All members do not have the same chance of
being selected
Purposeful Sampling Method
 Appropriate for:




Exploratory Research
Focus Groups
Organizing Committees
Building Networks
 Do not use if your goal is to learn about the larger
population
–Results are not generalizable!!
Example 1:
Successful Hunting Experience
 Target Population: 1 million Deer Hunters
 Survey Objectives: Learn about Hunting
Success
 Acceptable Sampling Error: + 3%
 Confidence Level: 95%
 Population Variation: 80/20
Example 1:
Successful Hunting Experience
 Population List: Automated Licensing
Database
 Sample Size: 683
 Sampling Method: Systematic Sampling
Example 2:
Female Hunter Perceptions
 You know you want to study female
waterfowl hunters and their satisfaction
levels.
 But…


You do not know much about them
AND
You do not know your survey objectives
Example 2:
Female Hunter Perceptions
 Target Population: Female Waterfowl
Hunters (n = 100?)
 Survey Objectives: Explore their satisfaction
levels
 Acceptable Sampling Error: + 3%
 Confidence Level: 95%
 Population Variation: 50/50 or 80/20 ???
Example 2:
Female Hunter Perceptions
 Source: Automated Licensing Database
 Sample Method: ??
 Sample Size:??
Example 2:
Female Hunter Perceptions
The Solution!
 Conduct Exploratory Research
 Increase Sampling Error to + 10%
 Hold a series of focus groups (n = 50)
Example 2:
Female Hunter Perceptions
 Enables you to…




Learn about Population Variation
Learn about General Characteristics
Define Survey Objectives
Select appropriate sample size to be surveyed.
Review
 What does sample size depend on?
 What are the 3 steps to sampling?
 Does size matter?
Sources
 Dillman, D.A. 2000. Mail and Internet
Surveys: The Tailored Design Method. John
Wiley & Sons, Inc. 464pp.
 Salant, P. and D.A. Dillman. 1994. How to
Conduct your Own Survey. John Wiley &
Sons, Inc. 232pp.