Workshop 3

Question 1

Define risk

Answer 1

The probability that an event will occur in a group

Question 2

Define probability

Answer 2

an estimate of the chance of an event occurring divided by the total number of chances available. E.g. a player owns 1 of 4 tickets so probability of winning is ¼

Question 3

Define relative risk

Answer 3

probability of an event occurring in one group divided by the probability of an event occurring for a second group e.g. placebo vs treatment group

Question 4

Define odds

Answer 4

ratios of a player’s chances of losing against her chances of winning e.g. if a player owns 1 of 4 tickets odds would be 3:1

Question 5

What are the three measures of risk?

Answer 5

1. RR: probability of an outcome in an exposed group compared to the probability of an outcome in an unexposed group (smokers and non-smokers)
2. OR: measure of association between exposure and outcome (smoking and cancer)
3. AR: Individual risk of developing a disease over a time period

Question 6

Write the equation for RR

Answer 6

probability of event in treatment group/ probability of event in control group
A/ (A+B) / C/ (C+D)

Question 7

There are three different outcomes to the RR calculation. Explain them

Answer 7

• RR=1: exposure is not associated with the outcome
• RR>1: exposure is associated with increased outcome
• RR<1: exposure is associated with decreased outcome

Question 8

How is the OR calculated?

Answer 8

Odds ratio= odds of exposure in people with outcome/ odds of exposure in people without outcome
A/B / C/D = AD/ BC

Question 9

What is the 95% confidence interval?

Answer 9

The range of values in which you can be 95% certain contains the true mean of the population.

• CI shouldn’t include 1 to be statistically significant
• the larger the CI the lower the level of precision of the RR/OR

Question 10

When is RR used and when is OR used?

Answer 10

OR: rare, retrospective (case-control) studies
RR: common, prospective studies

Question 11

What is absolute risk? How is it calculated?

Answer 11

The probability or chance of an event occurring. Calculated by dividing the number of events in the group by the number of people in the group

Question 12

How is ARR calculated?

Answer 12

AR unexposed- AR exposed

Question 13

How is RRR calculated?

Answer 13

(AR unexposed – AR exposed)/ AR exposed

Question 14

Which of ARR and RRR is used for individual patients and why?

Answer 14

ARR because RRR would overestimate the individual risk