1. Day 3
Prevalence 1

2. Continuing with Tools for Doing the Study
On an average school day, how many hours do you watch TV?
A. I do not watch TV on an average school day
 B. Less than 1 hour per day
 C. 1 hour per day
 D. 2 hours per day
 E. 3 hours per day
 F. 4 hours per day
G. 5 or more hours per day
✔ From questions to answers
✔ From answers to counts
From counts to prevalence
From prevalence to statements

3. Continuing with tools for doing a study
✔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

4. Three main tools

5. Review - Tool # 1
Cross-sectional study design: a relatively quick way to test a hypothesis
Sometimes called a prevalence study
A snapshot of what is going on
This is the best design for a high school project, because it is feasible in a shorter time frame and can be conducted in classes. It might not have the most thorough information collected, but is an appropriate way to start exploring a hypothesis. Has limitations that will test students ability to think critically.
One point in time
An observational study

6. Review - Tool # 2
2. Contingency table: puts numbers in a table so we can get from answers to counts
Shows exposure and outcome
The simplest table is the 2x2 table
Everyone is in the table somewhere
Handy for calculations

7. Tool # 3
3. Prevalence – calculations to quantify outcomes in populations; prevalence ratios (comparisons) provide a measure of association between exposure and outcome
Calculated as a fraction or percentage
Everyone with the outcome – recent and long-term
Especially used in cross-sectional studies

8. A cross-sectional study is sometimes called a prevalence study.
From Epi Textbooks
The main outcome measure obtained from a cross-sectional study is prevalence.
A cross-sectional study is sometimes called a prevalence study.

9. Prevalence
The number of people with a specified condition or event, among a specified population and at a specified time
The proportion of a population found to have a condition (typically a disease such as diabetes or a health-related behavior such as smoking or seat-belt use)

11. Express it in numbers
The Numerator is the number of people in the population or sample who experienced the outcome or effect, in this case, wearing blue.
The Denominator is the total number of people in the population or sample, in this case, total number of students in the class.

12. Prevalence of wearing blue
Numerator
The number of students who are wearing blue
Denominator
All the students in the class

13. Prevalence of wearing blue
# in class
x 100
= % wearing blue
= Prevalence

14. Prevalence of wearing glasses
The number of students who are wearing glasses
Numerator
Denominator
All the students in the class

15. Prevalence of wearing glasses
# in class
x 100
= % wearing glasses
= Prevalence

16. Numerator Denominator
The number of students who had cereal for breakfast
Denominator
All the students in the class

17. Numerator Denominator The number of students who walked to school
All the students in the class

18. Numerator Denominator The number of students who . . . ?
All the students in the class

19. Prevalence Ratio A comparison of two prevalences
Calculated by dividing the prevalence of the outcome in the exposed by the prevalence of the outcome in the unexposed  
a/(a+b) divided by c/(c+d).

20. High school students who send more text messages/day are more likely to binge drink compared to students who send fewer text messages/day.
Binge drinker
Not a binge drinker
Prevalence Ratio
Prevalence
Total 30
Hyper-texter 30 70
100
30 %
100
a b
c d
1.4
Not a hyper-texter 88 88
312
400
22 %
400 ÷ a
a+b c
c + d
People who ____________________________________________
are ______ times as likely to _______________________________
compared to people who __________________________________

21. High school students who send more text messages/day are more likely to binge drink compared to students who send fewer text messages/day. 30
100
30 % 70
a b
c d
Hyper-texter
Not a hyper-texter
Total
Binge drinker
Not a binge drinker
Prevalence 88
400
22 %
312
Prevalence Ratio
1.4
Hyper-texters are 1.4 times as likely to binge drink than those who are not hyper-texters.

22. Interpretation of Prevalence Ratios
Results
Interpretation
Prevalence Ratio Above 1.0
POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group

23. Teenagers who are not restricted from watching R-rated films are more likely to try smoking compared to teenagers who have restrictions on watching R-rated films.
Tried smoking
Did not try smoking
Prevalence Ratio
Prevalence
Total
656
No restriction
656
771
1427
46 %
1427
a b
c d
3.5
Partial or complete restriction
413
413
2704
3117
13%
3117 ÷ a
a+b c
c + d
People who ____________________________________________
are ______ times as likely to _______________________________
compared to people who __________________________________

24. Teenagers who are not restricted from watching R-rated films are more likely to try smoking compared to teenagers who have restrictions on watching R-rated films.
656
1427
46 %
771
a b
c d
No restriction
Partial or complete restriction
Total
Tried smoking
Did not try smoking
Prevalence
413
3117
13%
2704
Prevalence Ratio
3.5
Teenagers who have no restrictions on watching R-rated films are 3.5 times as likely to try smoking as those who have restrictions.

25. Interpretation of Prevalence Ratios
Results
Interpretation
Prevalence Ratio Above 1.0
POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group

26. Students living in urban areas engage in more experimenting with prescription drugs than students living in rural areas.
Did Experiment
Did not experiment
Prevalence Ratio
Prevalence
Total 95
Urban 95
905
1000
9.5 %
1000
a b
c d
0.73
Rural
130
130
870
1000
13.0 %
1000 ÷ a
a+b c
c + d
People who ____________________________________________
are ______ times as likely to _______________________________
compared to people who __________________________________

27. Students living in urban areas engage in more experimenting with prescription drugs than students living in rural areas. 95
1000
9.5 %
905
a b
c d
Urban
Rural
Total
Experiment
Did not experiment
Prevalence
130
13.0 %
870
Prevalence Ratio
0.73
Students in urban areas are 0.73 times as likely to experiment with prescription drugs than students in rural areas.

28. Interpretation of Prevalence Ratios
Results
Interpretation
Prevalence Ratio Above 1.0
POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group

29. Interpretation of Prevalence Ratios
Results
Interpretation
Prevalence Ratio Above 1.0
POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group
Prevalence Ratio Below 1.0
NEGATIVE ASSOCIATION the prevalence rate among the exposed group is lower than the prevalence rate among the unexposed group

30. Interpretation of Prevalence Ratios
Results
Interpretation
Prevalence Ratio Above 1.0
POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group
Prevalence Ratio Below 1.0
NEGATIVE ASSOCIATION the prevalence rate among the exposed group is lower than the prevalence rate among the unexposed group
Prevalence Ratio At or Near 1.0
NO ASSOCIATION – the prevalence rate among the exposed group is similar or the same as the prevalence rate among the unexposed group
The interpretation of a prevalence ratio is:
A prevalence ratio of 1.0 means that the outcome of interest occurs with the same frequency in the exposed and unexposed groups
A prevalence ratio above 1.0 means that the outcome of interest occurs more among the exposed group than among the unexposed group
A prevalence ratio below 1.0 means that the outcome of interest occurs less in the exposed group than among the unexposed group
Also try to understand the strength of a study result. In other words, the further a prevalence ratio is from 1.0 (higher or lower), the stronger the association. For example, 1.1 is a weak positive association and 3.1 is a strongly positive association. Similarly, 0.95 is a weak negative association, while 0.45 is a strongly negative association. When an association is strong, we have more confidence that it is real.
A prevalence ratio of 1.1 is a weak positive association, while a prevalence ratio of 3.1 is a strong positive association
A prevalence ratio of 0.95 is a weak negative association, while a ratio of 0.45 is a strong negative association

31. Results from some Epi Teams in Paterson NJ
Epi Stars - Drinking at least 2 cans or a 20-ounce bottle of non-diet soda every day leads to a crash (feeling tired) - PR = 2.5
Pop Science – A healthy breakfast is associated with playing in an organized sport - PR = 0.96
Hypertensions – Receiving a daily, weekly, or monthly allowance is related to eating junk food/unhealthy food more than twice a day - PR = 1.6
Dr. Observation – Healthy eating (at least 2 servings of fruit and vegetables a day) results in better grades (“doing well in school”) - PR = 1.0

32. Quick Summary of Cross-Sectional Study Calculations
Questions about exposure and outcome are answered simultaneously.
Answers on exposure and outcome can be put into a 2x2 table.
A “yes/no” answer will fit
If using a multiple choice question, a predetermined “cut point” is needed to define a “higher/lower” range to fit into a 2x2 table.
Counts in the 2x2 table allow calculations of prevalence
Comparisons of prevalences (prevalence ratio) allows a statement about the association between exposure and outcome.

33. No difference between exposed and unexposed
Measure of Risk
The prevalence ratio (PR) is a measure of risk used in cross-sectional studies. It compares prevalence in the exposed to prevalence in the unexposed. A ratio of 1.0 denotes no difference between the two groups.
Interpreting PR and Confidence Intervals
Prevalence Ratio
No difference between exposed and unexposed
Examples
This slide is about statistical significance, which is beyond the scope of the Epi Challenge but may be of interest to some students.
The vertical red line indicates a PR of 1 (unity) = no difference in outcome among the exposed versus the unexposed.
The dark horizontal lines show the calculated PR (dot) and the width of the 95% confidence interval (95% probability that this interval contains the true SMR)
Any line that is completely to the left or to the right of the red bar is a statistically significant result at p<0.05

34. Breakout Assignment Prevalence

35. Perform a few practice calculations as needed

36. Deck Worksheet – page 2
Calculate the prevalence of the outcome – for the exposed group and for the unexposed group.
Calculate the prevalence ratio.
Populate the 2x2 table on page 1 with the above information.
Make a statement that uses the prevalence ratio to describe size of the association.

37. Study Proposal: Section 6
Data Analysis Plan
6a. Contingency Table
6b. Prevalence among exposed
6c. Prevalence among unexposed
6d. Prevalence Ratio
6e. Statement of results
6f. How prevalence ratio will be used in your study