Odds & Odds Ratio, Incidence & Prevalence

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

Define the odds ratio

Answer 2

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

Question 3

Odds ratios are the usual reported measure in what studies?

Answer 3

Case control studies

Question 4

Probability vs odds:

Answer 4

Probability is the fraction of times you’d expect to see an event in many trials. When expressed as a single number probability is always between 0 and 1.

So, if we take the example of rolling a dice:

• the probability of rolling a six is 1/6 or 0.166666
• the odds of rolling a six is 1/5 or 0.2

Question 5

In a trial comparing the use of paracetamol for dysmenorrhoea compared to placebo:

• 60 patients received paracetamol and 40 had 50% pain relief
• 90 patients received placebo and 30 had 50% pain relief

What is:

a) The odds of achieving significant pain relief with paracetamol?
b) The odds of achieving significant pain relief with placebo
c) The odds ratio?

Answer 5

a) 40 (achieved) / 20 (total - achieved) = 2

b) 30 (achieved) / 60 (total - achieved) = 0.5

c) 2/0.5 = 4

Question 6

Define incidence

Answer 6

The number of new cases per population in a given time period.

For example, if condition X has caused 40 new cases over the past 12 months per 1,000 of the population the annual incidence is 0.04 or 4%.

Question 7

Define prevalence

Answer 7

The TOTAL number of cases per population at a particular point in time.

Question 8

What 2 types can prevalence be divided into?

Answer 8

1) point prevalence

2) period prevalence

Question 9

Define point prevalence

Answer 9

The number of cases in a defined population / number of people in a defined population at the same time.

Question 10

Define period prevalence

Answer 10

The number of identified cases during a specified period of time / total number of people in that population.

Question 11

A questionnaire is sent to 2,500 adults asking them how much they weigh.

From this sample population, 500 of the adults were obese.

What is prevalence of obesity?

Answer 11

500/2500 = 0.2 or 20%

Question 12

Incidence vs prevalence in acute disease?

Answer 12

Incidence > prevalence

Question 13

Incidence vs prevalence in chronic disease?

Answer 13

Prevalence > incidence