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Chapter 12
Basic Sampling Issues
Carl McDaniel, Jr.
Roger Gates
Slides Prepared by
Bruce R. Barringer
University of Central Florida
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-1
Learning Objectives
Slide 1 of 2
• To understand the concept of sampling.
• To learn the steps in developing a sampling
plan.
• To understand the differences between
probability samples and nonprobability
samples.
• To understand the concepts of sampling
error and nonsampling error.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-2
Learning Objectives
Slide 2 of 2
• To review the types of probability sampling
methods.
• To gain insight into nonprobability
sampling methods.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-3
Definitions of Important Terms
• Population or Universe
– The total group of people from whom
information is needed.
• Census
– Data obtained from or about every member of
the population of interest.
• Sample
– A subset of the population of interest.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-4
Steps in Developing a Sample Plan
Step 7: Execute
Operational Sampling
Plan
Step 6: Develop and
Specify Operational
Plan
Step 2: Choose
Data Collection
Method
Step 1: Define the
Population of
Interest
Step 5: Determine
Sample Size
Step 3: Choose
Sampling Frames
Step 4: Select a
Sampling Method
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-5
Steps in Developing a Sampling Plan
Slide 1 of 8
• Step 1: Defining the Population of Interest
– Bases for defining the population of interest
include:
•
•
•
•
Geography
Demographics
Use
Awareness
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-6
Steps in Developing a Sampling Plan
Slide 2 of 8
• Step 2: Choosing a Data Collection Method
– The selection of a data collection method has
implications for the sampling process.
• Step 3: Choosing a Sampling Frame
– Sampling frame
• List of population elements from which to select
units to be sampled.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-7
Steps in Developing a Sampling Plan
Slide 3 of 8
• Step 4: Selecting the Sampling Method
– Probability samples
• Samples in which every element of the population
has a known, nonzero probability of selection.
– Nonprobability samples
• Include the selection of specific elements from the
population in a nonrandom manner.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-8
Steps in Developing a Sampling Plan
Slide 4 of 8
• Step 4: Selecting the Sampling Method
(continued)
– Sampling error: The difference between the
sample value and the true value of the
population mean.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-9
Steps in Developing a Sampling Plan
Slide 5 of 8
Advantages of
probability samples
Disadvantages of
probability samples
- The researcher can be sure of
obtaining information from a
representative cross section of the
population of interest.
- They are more expensive than
nonprobability samples of the
same size in most cases. The
rules for selection increase
interviewing costs and professional
time must be spent in developing
the sample design.
- Sampling error can be computed.
- The survey results are projectable
to the total population.
- Probability samples take more time
to design and execute than nonprobability samples.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-10
Steps in Developing a Sampling Plan
Slide 6 of 8
Advantages of
nonprobability samples
- Nonprobability samples cost less
than probability samples. This
characteristic of nonprobability
samples may have considerable
appeal in those situations where
accuracy is not of critical
importance.
- Nonprobability samples ordinarily
can be conducted more quickly
than probability samples.
Disadvantages of
nonprobability samples
- Sampling error cannot be computed.
- The researcher does not know the
degree to which the sample is
representative of the population
from which it was drawn, but can
draw inferences.
- The results of nonprobability
samples cannot and should not be
projected to the total population; this
is not true in all cases.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-11
Steps in Developing a Sampling Plan
Slide 7 of 8
• Step 5: Determine Sample Size
– Once the sampling method has been chosen, the
next step is to determine the appropriate sample
size.
• Step 6: Developing Operational Procedures
for Selecting Sample Elements
– Involves determining whether a probability or
nonprobability sample is being used.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-12
Steps in Developing a Sampling Plan
Slide 8 of 8
• Step 7: Execution of the Sampling Plan
– The final step in the sampling process involves
execution of the operational sampling plan
discussed in the previous steps.
– It is important that this step include adequate
checking to make sure that specified procedures
are adhered to.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-13
Classification of Sampling Methods
Sampling
Methods
Probability
Samples
Systematic
Cluster
Nonprobability
Stratified
Simple
Random
Convenience
Judgment
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Snowball
Quota
Slide 12-14
Probability Sampling Methods
Slide 1 of 4
• Simple Random Sampling
– Is considered to be the purest form of
probability sampling. A probability sample is a
sample in which every element of the
population has a known and equal probability
of being selected into the sample.
Probability of Selection =
Sample Size
Population Size
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-15
Probability Sampling Methods
Slide 2 of 4
• Systematic Sampling
– Probability sampling in which the entire
population is numbered, and elements are
drawn using a skip interval.
Population Size
Skip Interval = Sample Size
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-16
Probability Sampling Methods
Slide 3 of 4
• Stratified Samples
– Stratified samples are probability samples that
are distinguished by the following procedural
steps:
• First, the original or parent population is divided
into two or more mutually exclusive and exhaustive
subsets (e.g., male and female).
• Second, simple random samples of elements from
the two or more subsets are chosen independently
from each other, either proportionately or
disproportionately.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-17
Probability Sampling Methods
Slide 4 of 4
• Cluster Samples
– In the case of cluster samples, the sampling
units are selected in groups. There are two basic
steps in cluster sampling:
• First, the population of interest is divided into
mutually exclusive and exhaustive subsets.
• Second, a random sample of the subsets is selected.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-18
Nonprobability Sampling Methods
Slide 1 of 3
• Convenience Samples
– Nonprobability samples used primarily because
they are easy to collect.
• Judgment Samples
– Nonprobability samples in which the selection
criteria are based on personal judgment that the
element is representative of the population
under study.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-19
Nonprobability Sampling Methods
Slide 2 of 3
• Quota Samples
– Nonprobability samples in which population
subgroups are classified on the basis of
researcher judgment.
• Snowball Samples
– Nonprobability samples in which selection of
additional respondents is based on referrals
from the initial respondents.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-20
Nonprobability Sampling Methods
Slide 3 of 3
• Convenience Samples
– Nonprobability samples used primarily because
they are easy to collect.
• Judgment Samples
– Nonprobability samples in which the selection
criteria are based on personal judgment that the
element is representative of the population
under study.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-21
Summary of Key Points
Slide 1 of 2
• The population, or universe, is the total
group of people in whose opinions one is
interested.
• A census involves collecting desired
information from all the members of the
population of interest.
• A sample is simply a subset of a population.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-22
Summary of Key Points
Slide 2 of 2
• Probability sampling methods are selected
in such a way that every element of the
population has a known, nonzero
probability of selection.
• Nonprobability sampling methods include
all methods that select specific elements
from the population in a nonrandom
manner.
© 1999 South-Western Publishing McDaniel Gates Contemporary Marketing Research, 4e
Slide 12-23