Fundamentals of epidemiology - an evolving text - Are you looking ...

Calculation of person-time in a stable, dynamic population
c
m c = new cases
c
c d = deaths
c
c d c m = migrants
N p S U S C E P T I
c
B L E S
c m d
c
d c
c
d d p = prevalent cases
c
p p p p p p p p p p p
Processes at work: Immigration of cases, noncases
Out-migration of cases, noncases
Death of cases, noncases
Development of new cases
(t = time interval)
Relationship of CI and ID
cases cases
ID = –––––––––– or ID = ––––––––
N0t
Nt
Both ID and CI are actually old acquaintances who have changed their outfits. When we calculated
life expectancy in the first topic, we used the terms death rate, hazard, cumulative mortality,
cumulative survival. ID is essentially the hazard, now applied to events other than death. CI is
essentially the cumulative mortality proportion, now applied to events of any variety. Both represent
different summary statistics from survivorship analysis (known in engineering as failure-time
analysis).
ID is the rate at which the size of the unaffected population is changing, relative to the size of the
unaffected population; CI is the proportion of the original population that has been affected by time
t. CI is a cumulative measure from a baseline time to a specific later point in time. CI estimates the
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www.epidemiolog.net, © Victor J. Schoenbach 5. Measuring disease and exposure - 107
rev. 10/15/2000, 1/28/2001, 8/6/2001