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Epidemiology is "the study of the distribution and determinants
of disease frequency" (Hennekens
and Buring, 1987). Epidemiological information is used to
plan and evaluate strategies to prevent illness and as a guide
to the management of patients in whom disease has already developed.
In a parallel
way we can consider each symptom as a disease, and study how often
it occurs, and why, in different groups of people. This is helpful
in better understanding the relevance of each symptom in a specific
disease, in studying its causal relationships, and in developing
and testing strategies to improve the management of symptoms.
For example,
epidemiological research into breathlessness can:
- Describe
the frequency and severity of breathlessness at different stages
of disease, or in different groups of patients;
- Categorize
different types of breathlessness;
- Describe
the different factors associated with increased breathlessness;
- Develop
predictive indicators of those people most likely to experience
breathlessness; and/or
- Compare
the outcomes of different treatments.
The first
step in achieving these objectives is to measure the frequency
of the symptom. Providing that an objective and reliable means
of measuring a symptom exists, a researcher could simply count
the number of patients with a symptom of interest. For example,
a Palliative Care Unit might be interested in knowing the number
of patients dying with severe breathlessness in the past month.
Such information, however, does not allow direct comparisons,
and has limited utility for an epidemiological approach. The measures
of frequency commonly used (incidence and prevalence) take into
account not only the number of patients with the symptom (the
numerator), but also the population from which the patients with
the symptom derive (the denominator).
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