The document discusses key concepts in experimental design and sampling methods. It covers types of studies like observational and experimental. It also discusses randomization and different sampling methods such as random, systematic, stratified, and cluster sampling. The goal is to obtain samples that accurately represent the overall population to minimize sampling errors and biases.

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Created by Tom Wegleitner, Centreville, Virginia
Edited by Olga Pilipets, San Diego, California
Design of Experiments

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Key Concepts
 Sample data must be collected in
an appropriate way, such as
through a process of random
selection.
 If sample data are not collected in
an appropriate way, the data may
be so completely useless that no
amount of statistical torturing
can salvage them.

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observing and measuring specific
characteristics of objects without
modifying them in any way.
Example: Gallop poll ( observing
through interviews)
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4. SlideSlide 4
 In 1855 John Snow discovered that cholera is a
waterborne infectious disease. Snow was a
physician in London.
 Some 20 years before Koch and Pasteur laid the
foundations of modern microbiology
 By observing the course of the disease, he
concluded that cholera was caused by a living
organism, which entered the body with water or
food, multiplied in the body, and made the body
expel water containing copies of the organism.
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 Snow explained the lag between infection and disease
—a matter of hours or days—as the time needed for
the infectious agent to multiply in the body of the
victim (poisons work instantly)
 cholera spread along the tracks of human commerce.
 Furthermore, when a ship entered a port where
cholera was prevalent, sailors contracted the disease
only when they came into contact with residents of the
port.
These facts were easily explained if cholera was an
infectious disease,
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apply some treatment and then
observe its effects on the subjects
Example: In the experiment designed to
test the Salk vaccine, 200,000 children
were given the actual Salk vaccine and
200,000 other children were given a
placebo.
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 Cross sectional study
data are observed, measured, and collected at
one point in time
 Retrospective (or case control) study
data are collected from the past by going back in
time
 Prospective (or longitudinal or cohort) study
data are collected in the future from groups (called
cohorts) sharing common factors
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8. SlideSlide 8
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obtain that sample using an appropriate
method, such as one based on randomness
Replication and Sample Size
 Sample Size
use a sample size that is large enough to see
the true nature of any effects

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 Random Sample
members of the population are selected in
such a way that each individual member has
an equal chance of being selected
Definitions
Simple Random Sample (of size n)
subjects selected in such a way that every
possible sample of the same size n has the
same chance of being chosen

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Random Sampling
selection so that each
individual member has an
equal chance of being selected

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Systematic Sampling
Select some starting point and then
select every k th element in the population

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Convenience Sampling
use results that are easy to get

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Stratified Sampling
subdivide the population into at
least two different subgroups that share the same
characteristics, then draw a sample from each
subgroup (or stratum)

14. SlideSlide 14
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Cluster Sampling
divide the population into sections
(or clusters); randomly select some of those clusters;
choose all members from selected clusters

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 Random
 Systematic
 Convenience
 Stratified
 Cluster
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Methods of Sampling - Summary

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 Sampling error
the difference between a sample result and the
true population result; such an error results from
chance sample fluctuations
 Nonsampling error
sample data incorrectly collected, recorded, or
analyzed (such as by selecting a biased sample,
using a defective instrument, or copying the data
incorrectly)
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Definitions

17. SlideSlide 17
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Recap
In this section we have looked at:
 Types of studies and experiments
 Randomization
 Types of sampling
 Sampling errors

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#8 p. 31
Cruise ship passengers are given magnetic
bracelets, which they agree to wear in an attempt
to eliminate or diminish the effects of motion
sickness.
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19. SlideSlide 19
#10 p.31
A researcher from Johns Hopkins University obtains
data about the effects of alcohol on driving by
examining car crash reports from the past five
years.
retrospective
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#12 p.31
An economist collects income data by selecting and
interviewing subjects now, then going back in time
to see if they had the wisdom to take a statistics
course between the years of 1980 and 2008.
retrospective
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21. SlideSlide 21
#22 p.32
A market researcher has partitioned all California
residents into categories of unemployed,
employed full time and employed part time. She
is surveying 50 people from each category.
Stratified
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22. SlideSlide 22
#16 p.32
An engineering student measures the strength of
fingers used to push buttons by testing family
members.
Convenience
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#28 p.33
A researcher for the Orange County Department of Motor Vehicles
plans to test a new online driver registration system by using a
sample consisting of 20 randomly selected men and 20 randomly
selected women. (Orange County has an equal number of male and
female drivers). Does this sampling plan result in a random
sample? A simple random sample?
Yes, no. All drivers have the same chance of being selected, but
some samples (such as those with 21 men and 19 women) have no
chance of being chosen.
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