1. Steps to Creating A Research Question (Hypothesis):
Brainstorm possible risk factors & protective factors
Choose one (or a few) ‘factors’ to analyze
Predict the type of relationship between factors & outcome
Write your research question
Quantify your prediction in step 3 by…
Calculating the association between risk/protective factors & outcomes
Last time, we worked on brainstorming risk factors and creating a research questions from those RFs.
Today, our goal is to take it one step further:
SWBAT Calculate the association between risk/protective factors & outcome using a 2x2 contingency table.
The calculation we will learn today will help us determine how closely related the risk factor and outcome are. Since research in the real world is expensive and time consuming, it’s important to be sure you have evidence to back up your hunch before moving forward. Therefore, epidemiologists often try to measure associations before conducting a more complex study examining one factor.
1 m

2. Step 3: Predict the type of relationship between factor and outcome
Does _____________ cause _____________?
(Risk Factor)
(Outcome)
When you wrote the questions you just shared with your partner, your job was to rephrase them to more accurately depict the possible relationship you are looking for. We know that health problems are usually messy so there are many causes, and so our goal is to see how much each RF is associated with the OUTCOME. When you completed step 3, you probably ran into difficulties determining what type of relationship the RF & outcome had. But now we’ll learn how to quantify this relationship.
Think of it this way: Can we say those who do NOT get a vaccine will always get the flu? NO! Can we say that everyone who got the flu did NOT have a flu shot? NO! So we just want to find out how much the 2 are linked. Today, we’ll figure out how to MEASURE associations.
Remember why we try to avoid this word?
Today’s example:
Does not getting a vaccine cause influenza?
1 min

3. Is _____________ associated with _____________?
(Outcome)
(Risk Factor)
To figure this out we can compare incidence rates for people who:
Have or Do not have the Risk Factor
Have or Do not have the Outcome
For example:
Is not getting vaccinated associated with getting the flu?
We’ll us a 2x2 Contingency Table to sort all this out!
1st Say: “Notice how we changed the question at the top from CAUSE to ASSOCIATED WITH.” 2m

4. What goes into the 2x2 table?
Is _____________ associated with _____________?
(Outcome)
(Risk Factor)
What goes into the 2x2 table?
We just collect data on people based on RF & outcome.
Who had the FLU in the past year?
FLU: Go to the back of the room
NO FLU: Go to the front of the room
Who got a FLU SHOT in the past year?
FLU SHOT: Go to the door side corner
NO FLU SHOT: Go to the window side corner
You can also change this slide to just have them raise their hand. I had them move around for 2 of the

5. 2 x 2 Contingency Table Outcome Variable Risk Factor Variable
After the population is sampled (like we just did), we add the values into the table:
Flu shot/Flu (a)
Flu shot/No Flu (b)
No Flu shot/Flu (c)
No Flu shot/No Flu (d)
Outcome Variable
Risk Factor Variable
Non-honors: explain why it’s called 2x2 table:
Everyone yell out the answer to this question: what is 2 x 2? Ans: FOUR! So, when we have TWO yes/no variables we can collect data to group people into FOUR groups! That’s why we call this a 2x2 table.
What is the point of putting this data in a table? Well, it helps us organize the data so we can make some quick and important calculations to determine how ASSOCIATED the RF & OUTCOME are. So, first, it helps to add up the totals in each column and row (reveal animation uncovering totals in margins)
HAND OUT: Notes sheet to staple in notebook (begin stapler moving around room) 3m

6. 2 x 2 Contingency Table Let’s crunch some numbers!  Outcome Variable
Risk Factor Variable
QUICK REVIEW (non-honors): As scientists, we need to be very clear when we talk about rows and columns in a table!
When I say row, where am I looking? Which two numbers are in row 1? 35 and 5
When I say column, where am I looking? Which two numbers are in Column 1? 35 and 10 2m

7. Our big goal? Find RELATIVE RISK!
Relative Risk (RR): A ratio of the probability of the outcome occurring in the exposed (risk factor) group versus a non-exposed (no risk factor) group. RF
No RF 1m

8. Review: What is Incidence?
What does it mean?
New cases of OUTCOME (flu) in population over time
Doesn’t tell us anything about RF (flu shot) on it’s own
So we must compare 2 groups’ Incidence rates to learn how the flu shot might be linked to flu
What is the formula?
Look back at your notes!
Pull up projector screen to write formula on board (or use side board).
Show how TIME cancels (since it’s the same for both RF & NO RF group) when you divide a fraction by a fraction.
TIME cancels when we divide a fraction by a fraction because it is the same for the RF & NO RF group!

9. Calculating Relative Risk
If the incidence of the flu in the not vaccinated subpopulation is different from the incidence of the flu in the vaccinated subpopulation, then vaccination and getting the flu are observed to be ASSOCIATED.
Outcome Variable
Risk Factor Variable 1m

10. Calculating Relative Risk
Step 1: Focus on the risk factor row. Remember, not vaccinated is our risk factor.
Step 2: Calculate the incidence rate of the flu for those who are not vaccinated
incidence rate =
Outcome Variable
Important note:
Q: “What appears to be missing in this incidence calculation?” Refresh students on formula for incidence calculation (could be up on side board).
A: The bottom doesn’t show the time period!
Why? When we use incidence today, we are going to ignore the time period on the bottom, because we’ll be dividing an one incidence by another incidence that has the same time period, so the bottoms will cancel out! (optional: show this on the board)
Risk Factor Variable 3m

11. Calculating Relative Risk
Step 3: Focus on: no risk factor row. Remember, vaccinated is our no risk factor group.
Step 4: Calculate the incidence rate of the flu for those who are vaccinated
incidence rate =
Outcome Variable
Risk Factor Variable 2m

12. Put it all together to find RELATIVE RISK!
After we reach our final RR value (7.0 here), we need to ask: What does this number mean?
3 m

13. What does my RR value mean?
RR = 7.0? There is a VERY strong association (7x)
The RF group is 7 times as likely to experience the outcome.
RR = 1? There is no association.
The incidence in the RF group and no RF group were the same.
RR = 1.5? There is a strong association (1.5x)
The risk factor group is 1.5 times as likely to experience the outcome. (50% more likely)
RR = 1.1? There is some association (1.1x)
The risk factor group is 1.1 times as likely to experience the outcome. (10% more likely)
RR = 0.9? There is a inverse association
Exposure to risk factor decreases the risk of the outcome by 10%
*Often the relative risk will be between 1.0 and In order to determine whether or not a smaller relative risk is significant (which means the difference is greater than one would expect to observe by chance alone), more complex statistical calculations need to be done. But we won’t have time to go quite that far, so use your best judgment.
3 m

14. Focus on the decimal when RR is 1.0 – 1.99
.04 = 4%
When we look at a RR number, 1 is like our 0 point (no risk). So we only care about how much ABOVE 1 the RR is.
The RF group has 4% greater chance of experiencing outcome

15. Setting Up Your 2x2 Table
Our goal: Get RR ≥ 1. So we need the TOP to be larger than the bottom.
The top should represent:
NEGATIVE RISK FACTOR & BAD OUTCOME
BAD
NEGATIVE

16. Group Practice BAD NEGATIVE 10-15 m
Slides 1-12 should take about 25m total, leaving about 15 min. for practice. If they finish early, they can begin the homework.
10-15 m

17. Is _____________ associated with _____________?
(Outcome)
(Risk Factor)
What are we trying to prove by calculating the association (by getting a RR value)?
That the RF & outcome are RELATED or LINKED, not that there is a DIRECT CAUSE relationship
What other factor might people who get the flu shot and don’t get the flu have in common?
Health-conscious?
Hand-washers?
Less exposed to contagious environments?
More access to health care?
HONORS only, unless there is time in other classes:
Relative risk 2m