Odds Ratios

Question 1

What are odds?

Answer 1

A ratio of the number of people who incur a particular outcome to the number of people who do not incur the outcome.

Question 2

What is the odds ratio?

Answer 2

The ratio of the odds of a particular outcome with experimental treatment and that of control.

Question 3

What is probability?

Answer 3

The fraction of times you’d expect to see an event in many trials.

NOTE: When expressed as a number, probability will always between the numbers of 0 and 1.

Question 4

Give an example of the difference betwen probability and odds using rolling a dice.

Answer 4

The probability of rolling a six is 1/6 or 0.16666

The odds of rolling a six is 1/5 or 0.2

Question 5

How do you calculate odds ratio?

Answer 5

Trial comparing paracetamol to a placebo

Paracetamol group: Total number of patient 60 and only 40 percent achieved 50% pain relief.

Placebo group: Total number of patients 90 and only 30 achieved 50% pain relief.

THEREFORE

40 (The number who did achieve the outcome)/20(the number who didn’t achieve the outcome)= 2.

30/60= 0.5

Therefore the odds ratio = 2/0.5 = 4

NOTE: Odds ratio is always “number who achieved the outcome” on top and “number who didn’t achieve the outcome” on the bottom.

Question 6

What study are odds ratios used in?

Answer 6

Case control studies.

Question 7

If the outcome is rare, what does this mean for the odds ratio and the relative risk?

Answer 7

The odds ratio and relative risk will be similar.

As the prevalence of the disease or the outcome becomes rarer…the odds ratio drops to be closer to the RR.