Question 2. A study was conducted in Norway to assess the effect of body mass index (BMI) on the risk of adult mortality. One of the associations that study investigators were interested in was whether or not BMI values measured during adolescence were associated with an increased risk of mortality later in life. The table below is an abbreviated version of one of the published tables from this study.

BMI measured in adolescence (age 14-19)	No. of persons	No. of Deaths	No. of Person-Years
Low	25,067	325	240,169
Medium	82,985	1052	801,590
High	12,181	174	120,204
Very High	7,888	131	77,561

a. Create a 2 x 2 table to describe these data where your outcome of interest is the number of deaths and you are interested in estimating incidence rates and incidence rate ratios (e.g.; use person-time instead of the number of people).

b. Calculate the incidence rate of death among people with Low BMI, Medium BMI, High BMI, and Very High BMI.

c. Using the total study population, calculate the incidence rate ratio of death among people with (1) High BMI and (2) Very High BMI using Low BMI as the reference group.

d. Interpret each of the IRR you calculated above in words.

e. The authors of this study reported that the average amount of follow-up time per person was 9.7 years. How did they obtain this measure?