Measures of disease frequency

Question 1

Definition of prevalence

Answer 1

Proportion of population who have the disease at relevant time

Question 2

Formula for prevalence

Answer 2

Number of existing cases/total population

Question 3

What’s period prevalence

Answer 3

Proportion of population affected over a given period

Question 4

Definition of incidence

Answer 4

Number of new cases of disease occurring in a given population over a period of time

Question 5

Formula for incidence

Answer 5

Number of new cases during time period/ total population at risk

Question 6

Definition of risk

Answer 6

Probability that an event will occur

Question 7

Formula for risk

Answer 7

proportion who are diseases/total number exposed

Question 8

odds for disease given exposure formula

Answer 8

diseased from exposure/healthy from exposure

Question 9

What is relative risk

Answer 9

Odds ratio, hazard ratio, risk ratio, rate ratio, etc

Question 10

formula for risk ratio

Answer 10

exposed+diseased/total exposed

divided by

not exposed+diseased/total not exposed

Question 11

What are risk ratios and odds ratios like in rare diseases

Answer 11

very similar because number of diseased is very small

Question 12

How Is risk different worked out

Answer 12

Subtracting one risk from another

Question 13

What is NNT

Answer 13

Average number of patients who need to be treated to prevent one additional bad outcome

Question 14

Formula for NNT and example to show how its worked out

Answer 14

1/reduction in absolute risk

e.g. if theres 70% who die, but 40% who don’t,
its 1/0.3= 3.33
therefore 3.33 must be treated to save 1