5
Population/Universe
• Any complete group
– People
– Sales territories
– Stores
• Total group from which information is needed
8
Define the target population
Select a sampling frame
Conduct fieldwork
Determine if probability or non-probability sampling method will be chosen
Plan procedure for selecting sampling units
Determine sample size
Select actual sampling units
Stages in Selectinga Sample
9
Define Target Population
• Look at research objectives
• Relevant population
• Operationally define
• Consider alternatives and convenience
10
Select Sampling Frame
• List of elements from which sample may be drawn
• Mailing and commercial lists can be problematic (more on this later)
11
Sampling Units
• Group selected for the sample
• Can be persons, households, businesses, et cetera
• Primary sampling units
• Secondary sampling units
12
Choose Probability or Non-probability Sample
• Probability sample
• Known, nonzero probability for every element
• Non-probability sample
• Probability of selecting any particular member is unknown
18
Convenience Sample
• Also called haphazard or accidental sampling
• Sampling procedure for obtaining people or units that are convenient to researchers
21
Judgment Sample
• Also called purposive sampling
• Experienced person selects sample based on his or her judgment about some appropriate characteristics required of sample members
23
Quota Sample
• Various population subgroups are represented on pertinent sample characteristics to the extent desired by researchers
• Do not confuse with stratified sampling (discussed later)
25
Snowball Sample
• Initial respondents selected by probability methods
• Additional respondents obtained from information provided by initial respondents
26
Probability Samples
• Simple random sample
• Systematic sample
• Stratified sample
• Cluster sample
29
Stratified Sample
• Probability sample
• Sub-samples drawn within different strata
• Each stratum more or less equal on some characteristic
• Do not confuse with quota sample
36
Cluster Sample
• Purpose: to sample economically while retaining characteristics of a probability sample
• Primary sampling unit is not individual element in population
• Instead, it is larger cluster of elements located in proximity to one another
42
Bases for Choosing a Sample Design
• Degree of accuracy
• Resources
• Time
• Advanced knowledge of population
• National versus local
• Need for statistical analysis
43
After Sample Design is Selected
• Determine sample size
• Select actual sample units
• Conduct fieldwork
47
Random Sampling Error
• Difference between sample results and result of a census conducted using identical procedures
• Statistical fluctuation due to chance variations
49
Systematic Errors
• Non-sampling errors
• Unrepresentative sample results caused by flawed study design or imperfections in execution rather than chance
54
• Directories not current
• Demographics and socioeconomics of voluntary non-list members differ from list members
• Solution
– Random digit dialing
– Add ‘1’ to listed number
Directories and Telephone Interviewing
58
Internet Sampling is Unique
• Internet surveys allow researchers to rapidly reach a large sample
• Survey should be kept open long enough so all sample units can participate
59
Advantages and Disadvantages
• Internet samples may be representative of target populations
– e.g., visitors to a Web site
• Hard to reach subjects may participate
• Major disadvantage
– Lack of PC ownership & Internet access among certain population segments
60
Web Site Visitors
• Unrestricted samples are clearly convenience samples
• Randomly selecting visitors• Questionnaire request randomly
"pops up" • Over-representing more frequent
visitors
62
Panel Samples• Typically yield high response rates
– Members may be compensated for time with sweepstake or small cash incentive
• Database on members
– Demographic and other information from previous questionnaires
• Select quota samples based on product ownership, demographics, lifestyle, or other characteristics