Lecture 8- Contingency tables

Question 1

What do we use contingency tables for?

Answer 1

To show the relationship between 2 categorical variables

Question 2

Draw the set up of a standard contingency table

Answer 2

look at lecture slides for results

Question 3

What is relative risk? + How is it calculated?

Answer 3

The ratio of two probabilities

RR =(a/(a+b))/(c/(c+d))

Question 4

What is risk difference ? + How do you calculate it?

Answer 4

The difference between two probabilities

a/(a + b) − c/(c + d)

Question 5

What is odds ratio? + How do you calculate it?

Answer 5

Ratio of two odds

(a/b) /(c/d) = ad/bc

Question 6

What is the convention when setting up a relative risk calculation?

Answer 6

The group “exposed” goes on top

Question 7

How is OR symmetric and what does this mean?

Answer 7

Because the OR doesn’t use the row or column totals, the odds
ratio is symmetric with respect to the rows and columns in the table. This
means there is no mathematical distinction between exposure and outcome
variables (can flip the variables either way)

Question 8

When should OR be used as opposed to RR?

Answer 8

-Quantifying associations between binary variables where there is no
“direction” e.g. alcohol consumption (Yes/No) and smoking (Yes/No), or
fever and Diarrhoea.
-Case control study, the denominator is controlled by the researcher (they choose the number of cases and controls) therefore RR has no meaning

Question 9

What happens if an outcome is rare for a cohort study in terms of OR or RR?

Answer 9

Both will be similar so doesn’t matter what is used