Question

Person-Years at Risk and Incidence Density Person-Years at Risk sum of the amount of time a participant is at risk for the outcome in a study. (# persons x at risk for 1 year + # persons x at risk for 2 years + # persons x at risk for 3 years, etc.) Incidence Density (IDL- new cases of disease or mortality in persons at risk/person-time at risk. Incidence Density in Not Exposed (IDE) Person-time at risk Incidence Density in Exposed (IDr Person-time at risk Incidence Density Ratio (IDR -also called Relative Risk) ID/ID Compute the IDR and RR (6 points) This is an 8-year cohort study to determine the risk of smoking on developing throat cancer Some of the cohort is a past or current smoker. Others are never smokers. None has throat cancer at the beginning of the study; some develop it during the study. Compute the person-years at risk, incidence density for the exposed and unexposed groups, and the incidence density ratio. Then, interpret the findings. Steps. Compute the Person-Years of smoking exposure using the table below. (Hint: add the years at risk of throat cancer (TC)- see examples). .Once they develop TC they are no longer at risk and their years in the study can't be counted. Add each person's years of exposure in the right column and enter the value in the box next to Total P-Y at Risk of Throat Cancer" Then count the P-Y at risk for the Current/Former Smokers (exposed) and the P-Y at risk for the Never-Smokers (not exposed) Using the Person-Years for each exposure group as the denominator and the number of TC recorded for each exposure group; compute the Incidence Density for TC risk in current/former smokers (exposed) (ID) & compute the incidence density for TC risk in never smokers (not exposed) (IDNE) The compute the Incidence Density Ratio (also call RR), divide the IDi by the IDsE. This value is interpreted the same as the Relative Risk.

Answer 1

5.

		Years at the risk of developing Throat Cancer								Years at risk
Person	Smoker	1	2	3	4	5	6	7	8
A	Yes	at risk	at risk	TC	no longer at risk of Throat Cancer					2
B	No	at risk	at risk	at risk	at risk	at risk	at risk	at risk	at risk	8
C	No	at risk	at risk	at risk	at risk	at risk	at risk	at risk	at risk	8
D	Yes	at risk	at risk	at risk	at risk	at risk	TC	no longer at risk of Throat Cancer		5
E	Yes	at risk	at risk	at risk	TC	no longer at risk of Throat Cancer				3
F	No	at risk	at risk	at risk	at risk	at risk	at risk	at risk	at risk	8
G	No	at risk	at risk	at risk	at risk	TC	no longer at risk of Throat Cancer			4
H	No	at risk	at risk	at risk	at risk	at risk	at risk	at risk	TC	7
I	Yes	at risk	TC	no longer at risk of Throat Cancer						1
J	Yes	at risk	at risk	at risk	at risk	at risk	at risk	TC	no longer at risk of Throat Cancer	6
	Total P-Y at risk of Throat Cancer									52

6.Sum of Person Years of Exposure in each exposure group

Former/Current Smoker : 17

Never Smoker : 35

7. No of persons of exposed and non exposed participants who developed throat Cancer

Former/Current Smoker : 5

Never Smoker : 2

8.Incidence Density for the exposed (ID_E) and non-exposed (ID_NE) groups

Former/Current Smoker : 0.2941

Never Smoker : 0.0571

9.Incidence Density Ratio (IDR)

$IDR= \frac{ID_{E}}{ID_{NE}} = \frac{0.2941}{0.0571} = 5.1471 \simeq 5.15$

IDR > 1, thus it is hazardous.

10. Smokers have 5.15 times the risk of throat cancer compared to the non-smokers