Module 2 Section 4

Question 1

What is Relative Risk?

Answer 1

• Relative Risk (RR) refers to how many times more likely it is that one group of people will become ill compared to another group. Risk is simply the cumulative incidence of being exposed to an illness. Groups can be defined by demographic factors (e.g. age-group, sex) and exposures to a suspected risk (e.g. lives within 1km of open water, does not live within 1 km of open water)

Relative Risk = a/(a+b) / c/(c+d)

Question 2

Relative Risk = a/(a+b) / c/(c+d)

Answer 2

a/(a+b)
The primary group (E+) normally refers to the group exposed to the risk.
a = primary group (E+) with the disease (D+)
b = primary group (E+) without the disease (D-)

c/(c+d)
The comparative group (E-) refers to the group not exposed to the risk.
c = the comparative group (E-) with the disease (D+)
d = the comparative group (E-) without the disease (D-)

Question 3

Using a Table for RR Calculations

Answer 3

It can be helpful to do relative risk calculations using a 2x2 table.

Question 4

Interpreting Relative Risk

Answer 4

• If RR<1
• The primary interest group has a lower risk of disease
• If RR=1
• The primary interest group has a higher risk of disease
• If RR>1
• The primary interest group has a higher risk of disease
• Before RR values can be interpreted, they need to be subjected to statistical tests of significance. Significance tests check whether or not the discrepancy between risks could have occurred by chance alone

Question 5

Odds Ratio

Answer 5

In the absence of information about the incidence of an entire population, you can calculate an odds ratio based on information about the primary group (E+) and the comparative group (E-). In certain circumstances, like when the outcome is rare or when you cannot quantify the at-risk population such as in retrospective case-control studies, the odds ratio is considered a reasonable approximation of the relative risk. The same 2x2 table can be utilized to calculate odds ratio.

Odds Ratio = a × d / b x c

Question 6

Calculating an Odds Ratio

Answer 6

An odds ratio is calculated to approximate relative risk when incidence information is missing.