A population of 160 healthy firemen was followed for development of COPD. After being followed for 5 years 15 developed COPD. Twenty firemen were followed for 2 years and they left for other jobs. The remaining who never developed COPD were followed for 10 years.
a. Calculate the number of person years of observation accrued by the population
b. Calculate the Incidence rate of COPD amongst firemen.