Measure of disease frequency

Suppose that you are following a group of children for the development of asthma over a one- year period. You identify 100 children on January 1st, screen them for asthma, and set up a monitoring program to check on their status on a monthly basis. Five children are considered prevalent cases because they were diagnosed with asthma before January 1st. Ten children develop asthma on March 1st and another ten children develop asthma on July 1st. Another 10 children who remain healthy were followed for six months and then were lost to follow-up. All of the remaining children did not develop asthma and were not lost to follow-up. Follow-up ended on December 31st.

· What was the prevalence of asthma on June 1st?

· What was the prevalence of asthma on September 1st?

· How many person-months of observation were accrued by this population?

· What was the incidence rate of asthma in this population for the one-year period?

Question 2 

A population of 1,000 people is monitored for a year for the development of measles. No one has measles at the start of the investigation. Thirty people develop measles on June 30 and twenty people develop measles on September 30. Eight people are lost to follow-up on March 31 and twenty-four people are lost to follow-up on November 30. None of those lost to follow-up had developed measles prior to becoming lost. Assume that you can only get measles once.

· What is the cumulative incidence of measles in this population?

· What is the incidence rate of measles?

· What is the prevalence of measles on July 1?