Question: 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.

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

2. What was the prevalence of asthma on September 1st?

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

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