A population of 200 healthy fisherman was followed for development of Schistosomiasis. After being followed for 4 years 20 developed Schistosomiasis. 30 fishermen were followed for 6 years and they left for other jobs. The remaining who never developed Schistosomiasis were followed for 8 years.
a. Calculate the number of person years of observation accrued by the population.
b. Calculate the Incidence rate of Schistosomiasis amongst fishermen.