One hundred people over 60 years of age were studied for one year in a study of high blood pressure. Twenty had high blood pressure at the start of the study and a further 10 developed high blood...

One hundred people over 60 years of age were studied for one year in a study of high blood pressure.  Twenty had high blood pressure at the start of the study and a further 10 developed high blood pressure during the period. If you assume that high blood pressure is a chronic condition; that is, once you develop it you have it for good, what is the answer to the following questions:
4a. What is the prevalence of high blood pressure at the start of the period (2 marks)?
4b. What is the cumulative incidence of blood pressure (2 marks)?
4c. If you assume for your calculations that there were a total of 80 person years at risk in this

         study, calculate the incidence rate of blood pressure in this study. (2 marks)?
Explain the difference between cumulative incidence and incidence. In what situation would you prefer one of these measures of disease frequency over the other (2 marks)