Question

2,000 men were enrolled in a study. They were followed by investigators for 4 years. Each year, the men were tested for prostate cancer. After yr 1 , there were no cases, but 20 men were lost to follow up. After 2nd year, there were 10 cases of cancer and 25 men were lost to follow up. At the end of the 3rd year, there were 16 cases and 3 men were lost. At the end of the 4th year, there were 25 cases &30 men lost to follow up. What is person-time incidence rate of cancer in this group?

Answer 1

Solution:

Person-time incidence rate is calculated using the below formula.

Person-time incidence rate=Number of new cases during the period/ Time each person was observed,totaled for all person

The persons lost to follow-up are, on average, disease-free for half the year, and thus contribute ½ year to the denominator. Therefore, the person followed for one year before being lost to follow-up contributes 1.5 person-years. The same assumption is made for participants diagnosed with the disease at the year 2 examination. So, on average, they developed illness halfway through the year. As a result, persons diagnosed with the disease contribute ½ year of follow-up during the year of diagnosis.

The denominator of the person-time rate is the sum of all of the person-years for each study participant. So, someone lost to follow-up in year 3, and someone diagnosed with the disease in year 3, each contributes 2.5 years of disease-free follow-up to the denominator.

	Year 1	Year 2	Year 3	Year 4	Total
Number of new cases during the period	0	10	16	25	51
Time each person was observed,totaled for all person	1980+(1/2*20)	1945+(1/2*10)+(1/2*25)	1926+(1/2*16)+(1/2*3)	1871+(1/2*25)+(1/2*30)
	1990	1962.5	1935.5	1898.5	7786.5

From the above table let us calculate the Person-time incidence rate=51/7786.5=0.0065 cases per person year.