Risk and relative risk

Absolute risk

• Risk is simply the probability of an outcome for a group — the proportion of that group who have the outcome.
• Absolute risk of an outcome in a group:

$\text{risk} = \frac{\text{number with the outcome}}{\text{total in the group}}$

• Like any probability it is between 0 and 1, and can be written as a fraction, decimal or percentage.

Relative risk

• Relative risk compares the risk in one group with the risk in another, as a ratio (a division):

$\text{relative risk} = \frac{\text{risk in group A}}{\text{risk in group B}}$

• The group on the bottom is the baseline (comparison) group.
• Method: find each group's absolute risk first, then divide one by the other.

Interpreting relative risk

• Relative risk $= 1$ → the two groups have the same risk.
• Greater than 1 → group A is more at risk (a value of 2 means "twice as likely").
• Less than 1 → group A is less at risk.
• Always state it in words and name the comparison group, e.g. "group A is twice as likely as group B".