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The classic descriptors of how common a disease, symptom, or problem
is in a population are incidence and prevalence.
Incidence
quantifies the number of new problems that develop in a population
at risk during a given period of time (for example, new symptoms
such as new instances of breathlessness). Incidence is quantified
as a RATE:
incidence=
| number
of new problems during a given period of time |
| total
population (or person-time) at risk |
Incidence
provides an estimate of the probability (or risk) that a patient
will develop the problem during the given period of time.
For example:
At admission, 120 patients free from breathlessness are followed
for 3 weeks. In this period of time, 45 developed breathlessness.
This results in a incidence rate of 45 per 120 (or 37.5 %) during
the 3 week period. This means that, at admission, each patient
had a .375 probability to develop breathlessness during the next
3 weeks.
Prevalence
quantifies the proportion of a given population with a problem
(for example, breathlessness) at a designated time. When the term
is used without qualification of a time period it is usually the
point prevalence, for example the proportion of patients with
breathlessness at a given point in time. Note that, although the
term "prevalence rate" is used for prevalence, prevalence
is a proportion, not a rate.
prevalence=
| number
of patients with the problem | at
a designed time |
| total
population |
For example:
At admission, 150 patients were screened for the presence of breathlessness,
and 30 resulted affected by the symptom. This results in a breathlessness
prevalence at admission of 20% (30/150).
The period
prevalence refers to the proportion of a given population with
a problem (for example, breathlessness) at any time during a specified
period. Period prevalence can refer to proportion with the problem
at any time in one year (annual prevalence) or at any time in
their life (lifetime prevalence).
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