Cohort studies

Question 1

Define incidence risk

Answer 1

the probability that a person initially free from (and at risk of) the disease develops it at some time during the period of observation.
The both numerator and denominator are numbers of people

Question 2

Define incidence rate

Answer 2

(incidence density) is the rate of contracting the disease among those still at risk; when a person contracts the disease they are no longer at risk.
The numerator is a number of people but denominator is person years at risk (p-year).