You conduct a prospective cohort study to see if watching television is associat

Question

You conduct a prospective cohort study to see if watching television is associated with colorectal cancer. You divide the exposed group into three categories: those who watch <1 hour per day (low), those who watch 1-4 hours a day (moderate), and those who watch >4 hours a day (high). You follow the participants for five years. Of the 5000 participants who did not watch TV, 25 participants developed colorectal cancer. Of the 2000 with low exposure, 10 people developed colorectal cancer. You observed 15 cases of cancer among the 2000 participants with moderate exposure. Finally, 10 people developed colorectal cancer among the 1000 participants with high exposure.

a) Calculate the cumulative incidence of colorectal cancer (per 1000) FOR EACH of the four groups, separately: no TV, low, moderate and high exposure.

b)Calculate the cumulative incidence ratio FOR EACH of the three TV-watching groups (low, moderate and high) compared to those who did not watch TV.

Explanation / Answer

(a) Cumulative incidence for

(i) No TV group = 25/5000 * 1000 = 5 per 1000

(ii) Low exposure = 10 * 1000/2000 = 5 per 1000

(iii) Medium Exosure = 15 * 1000/2000 = 7.5 per 1000

(iv) High exposure = 10 * 1000/1000 =10 per 1000

(b) Cumulative incidence ration

for

(i) Low Exposure = 5 per 1000/ 5 per 1000 = 1

(ii) Medium Exosure = 7.5 per 1000/ 5 per 1000 = 1.5

(iii) HIgh Exposure = 10 per 1000/ 5 per 1000 = 2

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