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Day 3
2x2 Table
1
What are you curious about?
From curiosity to a hypothesis
From a hypothesis to questions
From questions to answers
From answers to counts
From counts to prevalence
From prevalence to statements
Interpretation – Conclusions - Communication
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Three main tools
1. Cross-sectional study design: a relatively quick way to test a hypothesis
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An observational study
A snapshot of what is going on
Sometimes called a
prevalence study
One point in time
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LA NY
Cross-sectional studies are carried out to investigate possible associations between hypothesized exposures and outcomes.
People in the study take a survey and give answers to questions about exposure and about outcome at the same time.
From Epi Textbooks
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What are you curious about?
From curiosity to a hypothesis
From a hypothesis to questions
From questions to answers
From answers to counts
From counts to prevalence
From prevalence to statements
Interpretation – Conclusions - Communication7
Cross-sectional study design
What are you curious about?
From curiosity to a hypothesis
From a hypothesis to questions
From questions to answers
From answers to counts
From counts to prevalence
From prevalence to statements
Interpretation – Conclusions - Communication8
Contingency Table
2. Contingency table: puts numbers in a table so we can get from answers to counts
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Handy for calculations
The simplest table is the 2x2 table
Shows exposure
and outcome
From Epi Textbooks
A contingency table is used to display exposure and outcome data so associations can be measured.
- Categories of one variable are presented in rows and categories of another variable are presented in columns
- The table is used to show the number people who:
have or do not have the exposure and
have or do not have the outcome.
- The simplest contingency table is the 2x2 table
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TotalOn a sports
team
Not on a sports team
Caffeine
No Caffeine
Exposure
Outcome
a b
c d
2x2 Table – Say it with numbers
What people are in cell a?
cell b?
cell c?
cell d?
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Where are you?
TotalExposure
Outcome
c d
Is there a comparison group in this 2x2 table?
If yes, where are they?
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2x2 Table
On a sports team
Not on a sports team
Caffeine
No Caffeine
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Comparison Group
People who participate in a study and do not have the exposure of interest
People whose results are compared to the group that was exposed
TotalExposure
Outcome
Hypothesis: People who drink caffeine
are more likely to be on a sports team.
Which cell contains people who answer
“yes” to both questions?
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2x2 Table
On a sports team
Not on a sports team
Caffeine
No Caffeine
a
Epi Team Challenge
a
b
c
d
a b
dc
An Apple a Day Keeps the Doctor Away
Total
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a b
dc
An Apple a Day Keeps the Doctor Away
An Apple a Day
Infrequent Doctor Visits
No Apple a Day
Frequent Doctor Visits Total
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a b
dc
People who are not exposed
dc
An Apple a Day Keeps the Doctor Away
Total
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An Apple a Day
Infrequent Doctor Visits
No Apple a Day
Frequent Doctor Visits
Total
a b
dc
Study Links Physical Fitness to Academic Achievement
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Total
a b
dc
Study Links Physical Fitness to Academic Achievement
Lower Grades
Higher Grades
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Physically Fit
Not Physically Fit
Total
a b
dc
People who are exposed
a b
Study Links Physical Fitness to Academic Achievement
Physically Fit
Lower Grades
Higher Grades
Not Physically Fit
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a b
dc
Total
Kids Who Watch R-Rated Movies More Likely to Drink or Smoke
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a b
dc
R-Rated Movies
Total Drink or Smoke
Kids Who Watch R-Rated Movies More Likely to Drink or Smoke
Do Not Drink or Smoke
No R-
Rated
Movies
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a b
dc
People who are exposed and have the outcome
a
Total
Kids Who Watch R-Rated Movies More Likely to Drink or Smoke
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R-Rated Movies
Drink or Smoke
Do Not Drink or Smoke
No R-Rated Movies
a b
dc
Smoking Linked to Youth Eating Disorders
Total
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a b
dc
Smoking Linked to Youth Eating Disorders
Smoke
Eating Disorder
Do Not Smoke
No Eating Disorder Total
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a b
dc
People who are exposed and do not have the outcome
b
Smoking Linked to Youth Eating Disorders
Smoke
Eating Disorder
Do Not Smoke
No Eating Disorder Total
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a b
dc
Family Meals Are Good for Mental Health
Total
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a b
dc
Family Meals Are Good for Mental Health
Family Meals
Total
Good Mental Health
Poor
Mental Health
No
Family Meals
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a b
dc
People who are not exposed and do not have the outcome
d
Family Meals Are Good for Mental Health
Total
Good Mental Health
Poor
Mental Health
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Family Meals
No
Family Meals
a b
dc
Study Links Iron Deficiency to Math Scores
Total
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a b
dc
Study Links Iron Deficiency to Math Scores
Iron Deficiency
Poor Math
Scores
No Iron
Deficiency
Good Math
Scores Total
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a b
dc
People who do not have the outcome and are not exposed
d
Study Links Iron Deficiency to Math Scores
Poor Math
Scores
Good Math
Scores Total
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No Iron
Deficiency
Iron Deficiency
a b
dc
Proximity of Fast Food Restaurants to Schools and Adolescent Obesity
Total
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a b
dc
Proximity of Fast Food Restaurants to Schools and Adolescent Obesity
Obese Not Obese Total
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Fast Food Restaurant(s) Near SchoolNo Fast Food Restaurant(s) Near School
a b
dc
People who do not have the outcome
d
b
Proximity of Fast Food Restaurants to Schools and Adolescent Obesity
Obese Not Obese Total
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Fast Food Restaurant(s) Near SchoolNo Fast Food Restaurant(s) Near School
a b
dc
Total
Higher Risk of Obesity among Teens with More Video Game Playing
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a b
dc
More Video Game Playing
Obese
Less Video Game Playing
Not
ObeseTotal
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Higher Risk of Obesity among Teens with More Video Game Playing
a b
dc
Obese Not Obese Total
People who are not exposed and have the outcome
c
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Higher Risk of Obesity among Teens with More Video Game Playing
More Video Game Playing
Less Video Game Playing
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Yes / Yes
No / No
Yes / No
No / Yes
Cell a
Cell b
Cell c
Cell d
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Exposed
Not Exposed
OutcomeNo
Outcome Total
IMPORTANT NOTICE!
Remember that any of
the study types can
use the idea of a 2x2
table to explore a
possible association
between and exposure
and outcome
Breakout Assignment
2x2 table
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From answers to counts
Data into 2x2 Table
CDC
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Fifty-four students were asked to participate in a cross-sectional study to explore the hypothesis:
“Students who get more sleep are likely to be more physically active, compared to students who get less sleep.”
What is the exposure question?
What is the outcome question?
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Yes Exposure
Yes Outcome Total
No Outcome
No Exposure
a b
dc
Label the 2x2 table for the hypothesis
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High Sleep
High Physical Activity Total
Low Physical Activity
Low Sleep
a b
dc
Label the 2x2 table for the hypothesis
What is the exposure question?
What is the outcome question?
Low Sleep
High Sleep
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High Sleep
High Physical Activity Total
Low Physical Activity
Low Sleep
a b
dc
Label the 2x2 table for the hypothesis
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Deck Worksheet – Page 1 – Sort deck and fill out 2x2 table
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• For each survey, figure out which 2x2 table cell the respondent belongs (a, b, c, or d
• Write the correct letter of the cell in the upper left hand corner of each sheet
• Sort the 54 cards by cell letter
• Add up the numbers for each cell and fill in the 2x2 table accordingly
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Missing answers?
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Compare answers from the other
teams in your school.
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