representative sampling presented at the awds task force’s marketing workshop big sky, montana...
TRANSCRIPT
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
Sample Size for 95 Percent Confidence
+ 3 %
Sampling Error
+ 5 %
Sampling Error
50/50 Split
80/20 Split
50/50 Split
80/20 Split
100 92 87 80 71
500 341 289 217 165
1,000 516 406 278 198
5,000 880 601 357 234
Larger Populations
Population Size
Sample Size for 95 Percent Confidence
+ 3 %
Sampling Error
+ 5 %
Sampling Error
50/50 Split
80/20 Split
50/50 Split
80/20 Split
10,000 964 639 370 240
50,000 1,045 674 381 245
100,000 1,056 678 383 245
1,000,000 1,066 682 384 246
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.