Abigail Bret Cindy and Dennis are the only by Dr. Janet Forrester

Question 8 / 10: Abigail, Bret, Cindy and Dennis are the only four students living on the third floor of the

building. They hear about the sick students on the second floor and are concerned that

influenza will spread to their floor. Midterm exams are approaching and they want to

stay healthy, at least until after the exams are over. Abigail presents with influenza

symptoms on the morning of Monday, October 11th, one week before midterm exams

begin. Bret and Cindy decide to leave the dorm and stay with friends until midterm

exams are over. They leave first thing Tuesday morning, October 12th. Thursday

morning (October 14th) Dennis is showing influenza symptoms. Starting from Monday,

October 1st, what is the influenza incidence density for the third floor?

Answer:

The numerator is easy = 2 new cases of influenza (Abigail and Dennis).

It‘s the denominator that takes some thought:

Abigail contributes 10 person-days at risk before she gets sick (October 1thto 10th).

Bret and Cindy each contribute 11 person-days at risk (October 1st to 11th) before they leave the dorm.

Dennis contributes 13 person days before he gets influenza (October 1stto 13th)

So, the total number of person days for the denominator is: 10+2(11)+13 = 45

The influenza incidence density for the third floor is 2/45 person-days. (Note the time frame is embodied in the person-days)

You can understand why in large studies we leave the calculation to the computer!

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Descriptive Epidemiology Incidence Prevalence

Author:

Dr.Janet ForresterProfessor

Tufts University School of Medicine

USA

Access: Public Partially Instant Grading

Attribution: Janet, E.A. Forrester, Kwan Ho Kenneth Chui, Steven Cohen, Michael D. Kneeland, Alice Tang, David Tybor. Epidemiology and Biostatistics 2010. (Tufts University OpenCourseWare), http://ocw.tufts.edu/Course/65/ (Accessed 3 May, 2014). License: Creative Commons BY-NC-SA

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