Module 4 Overview
Context
Content Area: Interpretation of Epidemiological Evidence
Essential Question (Generic): Is the association causal?
Essential Question (Drug Abuse Specific): Is an association with drug abuse causal?
Enduring Epidemiological Understanding: Causation is only one explanation for finding
an association between an exposure and a disease. Because observational studies are
complicated by factors not controlled by the observer, other explanations must also be
considered.
Synopsis:
In Module 4, students explore the rationale and methods of interpreting epidemiological
studies. Students develop skills to assess possible explanations for an association
found in a study, with consideration of explanations of causality, chance, confounding,
reversed time order, and bias. Module 4 concludes with consideration of methods for
weighing the overall evidence for an association.
Lessons:
Lesson 4-1:
Lesson 4-2:
Lesson 4-3:
Lesson 4-4:
Lesson 4-5:
Lesson 4-6:
Lesson 4-7:
DrugEpi 4-3 Chance
Introduction to Interpreting Associations
Causality
Chance
Bias
Confounding
Reverse Time Order
Weighing the Evidence
Module 4 - Interpretation of Epidemiological Evidence
Lesson 4-3 Chance
Content
• A class exercise with 100 cards to illustrate
“chance” as a possible explanation for an association
Big Ideas
• When an association is found, several possible explanations must be
considered, including the possibility that the association is due to chance
• The larger the sample size, the less likely that an observed association is
due to chance
This project is supported by a Science Education Drug Abuse Partnership Award, Grant Number 1R24DA016357-01,
from the National Institute on Drug Abuse, National Institutes of Health.
DrugEpi 4-3 Chance
Where are we?
Essential Questions
Enduring Understandings
1.
How is this disease
distributed?
Health-related conditions and behaviors are not distributed uniformly
in a population. They have unique distributions that can be described
by how they are distributed in terms of person, place, and time.
2.
What hypotheses might
explain the distribution
of disease?
Clues for formulating hypotheses can be found by observing the way
a health-related condition or behavior is distributed in a population.
3.
Is there an association
between the
hypothesized cause and
the disease?
Causal hypotheses can be tested by observing exposures and
diseases of people as they go about their daily lives. Information
from these observational studies can be used to make and compare
rates and identify associations.
4.
Is the association
causal?
Causation is only one explanation for an association between an
exposure and a disease. Because observational studies are
complicated by factors not controlled by the observer, other
explanations also must be considered.
5.
What should be done
when preventable
causes of disease are
found?
Policy decisions are based on more than the scientific evidence.
Because of competing values - social, economic, ethical,
environmental, cultural, and political factors may also be considered.
DrugEpi 4-3 Chance
Chance
Is the
association due
to chance?
1.
Cause
2. Chance
3.
4.
5.
DrugEpi 4-3 Chance
Review
Population
All the people in a particular group.
DrugEpi 4-3 Chance
Review
Sample
A selection of people from a population
DrugEpi 4-3 Chance
Review
Inference
Process of predicting from what is observed
to what is not observed.
DrugEpi 4-3 Chance
Population
DrugEpi 4-3 Chance
Population
25 cards
25 cards
25 cards
25 cards
DrugEpi 4-3 Chance
Population
Exposure =
eating chocolate
=
DrugEpi 4-3 Chance
Outcome =
trying marijuana
Population
No
No
Marijuana Marijuana
=
DrugEpi 4-3 Chance
25 cards
25 cards
25 cards
25 cards
=
Odd #
a
b
Even #
c
d
Total
Review - The 2x2 Table
The 2x2 table
can express
Exposed
relationships
Not Exposed
between exposure
and outcome
DrugEpi 4-3 Chance
Outcome
No
Outcome
Total
Review - The 2x2 Table
The 2x2 table is a tool
used to express
the numbers of people
with and without the exposure
and with and without the outcome.
DrugEpi 4-3 Chance
Review - Another Way To Say . . .
A cross-classification of data
where categories of one variable
are presented in rows and
categories of another variable
are presented in columns.
DrugEpi 4-3 Chance
Review - The 2x2 Table
Outcome / Disease
E
x
p
o
s
u
r
e
Outcome
No
Outcome
Exposed
a
b
Not Exposed
c
d
DrugEpi 4-3 Chance
Total
Are there
controls in this
2x2 table?
If yes,
where are they?
Population
No
No
Marijuana Marijuana
=
DrugEpi 4-3 Chance
25 cards
25 cards
25 cards
25 cards
=
Total
Odd #
25
25
50
Even #
25
25
50
Population
=
=
=
25 cards
25 cards
25 cards
25 cards
No
No
Marijuana Marijuana
DrugEpi 4-3 Chance
Total
Odd #
25
25
50
Even #
25
25
50
Flu
No
Flu
Total
M&M’s
25
25
50
No
M&M’s
25
25
50
Review
Risk
Relative Risk
A measure of
how often
an event occurs
in a defined group
of people
in a defined
period of time
A way of showing the
relationship between two risks
The likelihood of
developing a disease
DrugEpi 4-3 Chance
Tells us the number of times one
risk is larger or smaller
than another
Calculated by dividing
the risk of an outcome
in one group
by the risk of the outcome
another group
in
Population
=
=
25 cards
25 cards
25 cards
25 cards
No
No
Marijuana Marijuana
DrugEpi 4-3 Chance
Total
Risk
Odd #
25
25
50
25 / 50 or 50%
Even #
25
25
50
25 / 50 or 50%
Population
=
=
25 cards
25 cards
25 cards
25 cards
No
No
Marijuana Marijuana
DrugEpi 4-3 Chance
Total
Risk
Odd #
25
25
50
25 / 50 or 50 %
Even #
25
25
50
25 / 50 or 50 %
Relative Risk
50 %
____
50 % =
/ 1
50% =
50 %
Population
25 cards
25 cards
25 cards
25 cards
DrugEpi 4-3 Chance
Definition
Chance
To occur accidentally
To occur without design
A coincidence
DrugEpi 4-3 Chance
Chance
DrugEpi 4-3 Chance
Chance
DrugEpi 4-3 Chance
Sample
25 cards
25 cards
25 cards
25 cards
DrugEpi 4-3 Chance
Sample
25 cards
25 cards
25 cards
25 cards
No
No
Marijuana Marijuana
DrugEpi 4-3 Chance
Total
Chocolate
5
5
10
No
Chocolate
5
5
10
Sample
25 cards
25 cards
25 cards
25 cards
No
No
Marijuana Marijuana
DrugEpi 4-3 Chance
Total
Risk
Chocolate
5
5
10
5 / 10 or 50 %
No
Chocolate
5
5
10
5 / 10 or 50 %
Sample
25 cards
25 cards
25 cards
25 cards
No
No
Marijuana Marijuana
DrugEpi 4-3 Chance
Total
Risk
Chocolate
5
5
10
5 / 10 or 50 %
No
Chocolate
5
5
10
5 / 10 or 50 %
Relative Risk
50 %
____
50 % =/ 1.0
50% =
50 %
Sample
No
No
Marijuana Marijuana
Chocolate
Total
Risk
5 / 10 = 50 %
Relative Risk
%
=
50 1
%
___
No
Chocolate
DrugEpi 4-3 Chance
5 / 10 = 50 %
Chance
No
No
Marijuana Marijuana
Chocolate
No
Chocolate
DrugEpi 4-3 Chance
5
5
5
5
Total
10
10
Risk
5 / 10 or 50 %
5 / 10 or 50 %
Relative Risk
50 %
____
50 %
= 1.0
Worksheet
DrugEpi 4-3 Chance
Chance
Relative Risks
Greater than 1.0
DrugEpi 4-3 Chance
Less than 1.0
In the News
DrugEpi 4-3 Chance
Possible Explanations for Finding an Association
Relative Risks
Greater than 1.0
DrugEpi 4-3 Chance
Less than 1.0
In the News
DrugEpi 4-3 Chance
Chance
20
card
s
Relative Risks
Greater than 1.0
DrugEpi 4-3 Chance
25 cards
25 cards
25 cards
25 cards
Less than 1.0
Different Sample Sizes
No
No
Marijuana Marijuana
Odd #
Total
Risk
5 / 10 = 50 %
Relative Risk
%
50
%
___
Even #
DrugEpi 4-3 Chance
5 / 10 = 50 %
=
Chance
50
card
s
Relative Risks
Greater than 1.0
DrugEpi 4-3 Chance
25 cards
25 cards
25 cards
25 cards
Less than 1.0
Different Sample Sizes
No
No
Marijuana Marijuana
Odd #
Even #
DrugEpi 4-3 Chance
Total
Risk
5 / 10 = 50 %
5 / 10 = 50 %
Relative Risk
%
___
50
%
=
Chance
75
card
s
Relative Risks
Greater than 1.0
DrugEpi 4-3 Chance
25 cards
25 cards
25 cards
25 cards
Less than 1.0
Different Sample Sizes
No
No
Marijuana Marijuana
Odd #
Total
Risk
5 / 10 = 50 %
Relative Risk
%
=
50 1
%
___
Even #
DrugEpi 4-3 Chance
5 / 10 = 50 %
Review Results
DrugEpi 4-3 Chance
Chance
Could the
association
have occurred
by chance?
1.
Cause
2. Chance
3.
4.
5.
DrugEpi 4-3 Chance
Re-Cap
Big Ideas in this Lesson (4-3)
•
When an association is found, several possible
explanations must be considered, including the
possibility that the association is due to chance
•
The larger the sample size, the less likely that an
observed association is due to chance
This project is supported by a Science Education Drug Abuse Partnership Award, Grant Number 1R24DA016357-01,
from the National Institute on Drug Abuse, National Institutes of Health.
DrugEpi 4-3 Chance
Next Lesson
Bias
DrugEpi 4-3 Chance