Week 5

Question 1

What is probability

Answer 1

the probability of the occurrence of an event is the
proportion of times that event would (or does) occur in a
large number of repeated trials.

Values ranges from 0(never happen) to 1 (always happen)

probability = proportion

Question 2

How to calculate risk?

Answer 2

D/N

Question 3

How to calculate risk difference?

Answer 3

risk difference = risk in exposed - risk in non exposed

Question 4

Risk ratio interpretation

Answer 4

RR = 1 occurs when there is no association between exposure
and disorder.

RR > 1 occurs when the risk is higher among the exposed

RR < 1 occurs when the risk is lower among the expose

The further the RR is from 1, the stronger the association
between exposure and disorder

nb a RR is always a positive number

Question 5

Can you estimate risk in case-control studies?

Answer 5

No, because, in case-control studies, the cases are an unknown
fraction of the population, it is not possible to estimate risk.

Instead, you can estimate odds of exposure by disorder.

Question 6

Interpretation of Odds Ratio

Answer 6

OR = 1 occurs when there is no association between exposure
and disorder.

OR > 1 occurs when the risk is higher among the exposed.

OR < 1 occurs when the risk is lower among the exposed.

The further the OR is from 1, the stronger the association
between exposure and disorder.

nb an OR is always a positive number

Question 7

Why use odds ratios?

Answer 7

common mistake is to interpret the odds ratio as if it were a
risk ratio

• this is ok for a rare outcome (i.e., probability of event not
occurring is close to 1), when the odds ratio approximates the
risk ratio (as the odds approximate the risk)