Geoscience Reference
In-Depth Information
Solution:
NumeratorNumberofnew cases of heartdise
=
ase
=+++=
017816
Denominator
=
= 2000
Person-yearsof observation
[
]
+
[
]
+
[
]
+×
(
1/2
100
)
1900
+ ×+ ×
(
1/2
1
)
(
1/2
99
)
1100
++×+×
(
1/2
7
)
(
1/2
793
)
[ ]
= 6400 person-yearsoffollow-up
++×+ ×
700
(
1/2
8
)
(
1/2
392
)
or
Denominator
=
=11..
Person-yearsof observation
(
)
+×
(
)
+×
(
)
+×
(
)
+×
(
)
+
(
)
5725
.
835
.
100
05
.
99
15
.
79
3325
×
.
(
)
+
(
)
+×
392
. .
person-yearsoffollow-up
35
700
×
40
=
6400
Number of new cases of dis
easeorinjury duringspecified period
Time each person was observed, totaledfor all persons
cases perperson-y
Person-time rate
=
=
16
/
6400
=
00025
.
ear
= 2.
cases per1000 person-years
In contrast, the incidence proportion can be calculated as 16/2100 = 7.6 cases per 1000 population
during the 4-year period, or an average of 1.9 cases per 1000 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 4 years.
■
EXAMPLE 8.9
Problem:
A diabetes follow-up study included 218 diabetic women and 3823 non-diabetic women.
By the end of the study, 72 of the diabetic women and 511 of the non-diabetic women had died.
The diabetic women were observed for a total of 1862 person-years; the non-diabetic women were
observed for a total of 36,653 person-years. Calculate the incidence rates of death for the diabetic
and non-diabetic women.
Solution:
For diabetic women:
Numerator 2
Denominator
=
=
1862
Person-time rat
e
=
=
= .. 6deaths per 1000 person-years
72/1862
0.0386 deaths perperson-year
38
Search WWH ::
Custom Search