Incidence & Prevalence

Look at the following table describing a 5-year study of a population of five patients. We are following these five healthy patients, as we would like to analyse the incidence of disease ‘A’.

	Year 1	Year 2	Year 3	Year 4	Year 5
Patient 1	Healthy	Healthy	Healthy	No data	No data
Patient 2	Healthy	‘A’	Healthy	Healthy	Healthy
Patient 3	Healthy	Healthy	Healthy	Healthy	Healthy
Patient 4	Healthy	Healthy	Healthy	‘A’	Healthy
Patient 5	Healthy	Died

Patient 1 was followed for three years and after his third annual check he did not show up any more. We do not know why. He did not get the disease ‘A’ in the first three years and we do not know what happened afterwards. We will keep him in our records with ‘3 person-years at risk’.

Patient 2 was followed for five years and got sick in the second year but recovered. How many years at risk will we include? One or four? This actually depends on the type of the disease. We assume in our example that you could get the disease more than once in your lifetime, so we would include him with ‘4 person-years at risk’.

Patient 3 has been followed for five years and never got the disease; he was ‘5 person-years at risk’.

Patient 4 has got the disease in the fourth year. We will include four years at risk into our calculation as we said earlier that you could get re-infected with the disease in this example.

Patient 5 was followed for one year and died (of an unknown cause). We do not know any details regarding disease ‘A’. Only the one year in which they were at risk (i.e. healthy) is added to the cumulative total of person-years at risk.

We found two new cases in a population of five persons, so we could say that the incidence was:

We might speak of an incidence of 40 new cases per 100 persons. We can also calculate the incidence using healthy years instead of total population. In this case we divide the number of new cases (two cases) by the total of the healthy years where persons are at risk to the disease (17 years).