EXAMPLES: Calculating Incidence Rates
Example A:
Investigators enrolled 2,100 women in a study and followed them annually for four years to determine the incidence rate of heart
disease. After one year, none had a new diagnosis of heart disease, but 100 had been lost to follow-up. After two years, one had a new diagnosis
of heart disease, and another 99 had been lost to follow-up. After three years, another seven had new diagnoses of heart disease, and 793 had been
lost to follow-up. After four years, another 8 had new diagnoses with heart disease, and 392 more had been lost to follow-up.
The study results could also be described as follows: No heart disease was diagnosed at the first year. Heart disease was diagnosed in one woman
at the second year, in seven women at the third year, and in eight women at the fourth year of follow-up. One hundred women were lost to follow-
up by the first year, another 99 were lost to follow-up after two years, another 793 were lost to follow-up after three years, and another 392
women were lost to follow-up after 4 years, leaving 700 women who were followed for four years and remained disease free.
Calculate the incidence rate of heart disease among this cohort. Assume that persons with new diagnoses of heart disease and those lost to follow-
up were disease-free for half the year, and thus contribute ½ year to the denominator.
Numerator
= number of new cases of heart disease
= 0 + 1 + 7 + 8
= 16
Denominator
= person-years of observation
= (2,000 + ½ x 100) + (1,900 + ½ x 1 + ½ x 99) + (1,100 + ½ x 7 + ½ x 793) +
(700 + ½ x 8 + ½ x 392)
= 6,400 person-years of follow-up
or
Denominator
= person-years of observation
= (1 x 1.5) + (7 x 2.5) + (8 x 3.5) + (100 x 0.5) + (99 x 1.5) + (793 x 2.5) +
(392 x 3.5) + (700 x 4)
= 6,400 person-years of follow-up
Person-time rate = Number of new cases of disease or injury during specified period
Time each person was observed, totaled for all persons
= 16 / 6,400
= .0025 cases per person-year
= 2.5 cases per 1,000 person-years

In
contrast,
the incidence proportion can be calculated
as 16 / 2,100 =
7.6 cases per 1,000 population during
the four-year period, or an average of 1.9 cases per 1,000 per year (7.6 divided by 4 years).
The incidence
proportion underestimates the true rate because it ignores persons lost to follow-up, and assumes that they
remained disease-free for all four years.
Example B:
The diabetes follow-up study included 218 diabetic women and 3,823 nondiabetic women. By
the end of the study, 72 of the diabetic women and 511 of the nondiabetic women had died. The diabetic
women were observed for a total of 1,862 person years; the nondiabetic women were observed for a total of
36,653 person years. Calculate the incidence rates of death for the diabetic and non-diabetic women.
For diabetic women,
numerator
= 72 and
denominator
= 1,862
Person-time rate = 72 / 1,862
= 0.0386 deaths per person-year
=
38.6 deaths per 1,000 person-years
For nondiabetic women,
numerator
= 511 and
denominator
= 36,653
Person-time rate = 511 / 36,653 = 0.0139 deaths per person-years

Example C
: In 2003, 44,232 new cases of acquired immunodeficiency syndrome
(AIDS) were reported in the United States.5 The estimated mid-year population of
the U.S. in 2003 was approximately 290,809,777.6 Calculate the incidence rate of
AIDS in 2003.

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