S2 - Measures of Association

Question 1

What two factors are compared in a two-by-two table?

Answer 1

Exposure (Yes or No) and Outcome (Yes or No).

‘Total’ columns are often added to this table.

Question 2

How is a risk ratio calculated?

Answer 2

Divide the risk of the outcome in the exposed (a / (a + b)) by the risk of the outcome in the unexposed (c / (c + d)).

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

Question 3

How is a risk difference calculated?

Answer 3

Subtracting the risk of the outcome in the unexposed (c / d) from the risk of the outcome in the exposed (a / b).

(a / (a + b)) - (c / (c + d)) = RD

Question 4

What three measures of association can be calculated from a two-by-two table?

Answer 4

Risk difference
Risk ratio
Odds ratio

Question 5

What is the difference between calculating a rate and calculating a risk?

Answer 5

A rate is an observed number of cases per unit time.

A risk is just a proportion.

Question 6

What is the difference between odds and risk ratios?

Answer 6

Odds ratios are more useful for case-control studies and/or rare diseases. This is more useful when we don’t have totals that represent the population (I.e. we’ve matched 500 cases for 500 controls).

Risk ratios are based on the total percentage in the group, and so we need somewhat representative groups to utilize RRs.

Useful link:
https://www.mdedge.com/jfponline/article/65515/relative-risks-and-odds-ratios-whats-difference

Question 7

How is an odds ratio calculated?

Answer 7

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

Basically, the odds of your having the outcome if exposed over the odds of your having the outcome if not exposed.