A population of 1,600 healthy, at risk people is monitored for one year starting on January 1st and the development of cases of chicken pox is noted. No one has chicken pox at the start of the investigation. Twenty people develop chicken pox on June 30th and forty people develop chicken pox on September 30th. Assume that you can get chicken pox only once. What is the cumulative incidence of chicken pox in this population during the one-year period from January 1st through December 31st?

3.75%

Explanation:

Cumulative incidence = Number of new cases of disease / the total number of at risk individuals for a period of time

From the question, no one has chicken pox at the start = 0

people developed chicken pox on June 30th = 20

people developed chicken pox on September 30th = 40

Number of new cases of disease = 0+20+40 = 60

At the end of the year, only 60 people would get chicken pox

Since there was no one lost to follow up, then the total population at the end of 1 year = 1600

Cummulative incidence rate = 60/1600 = 0.0375

This is equal to 3.75%

Or 37.5 out of 1000 people.