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Because
the aims of epidemiology are to estimate the proportion of people
with a certain condition (i.e. determine prevalence) or to enumerate
onsets of the condition (i.e. determine incidence) or to discover
risk factors for the condition, deciding who should be counted
as a case is central to conducting epidemiologic studies.
For some conditions,
case definition is relatively straightforward and relies on testing
a biological sample (e.g. a biopsy specimen or a bacterial culture)
or on observing specific clinical signs (e.g. fever with a characteristic
rash). However, for conditions where pain is a central defining
characteristic, a number of issues related to case definition
must be considered. These include issues related to measuring
pain by self report and deciding on thresholds for symptom severity
and duration.
Definition
of Pain
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| The
International Association for the Study of Pain defines
pain as "An unpleasant sensory and emotional experience
associated with actual or potential tissue damage or
described in terms of such damage" (Mersky
and Bogduk, 1994) |
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Since experience
is, by definition, individual and subjective, case definitions
of pain conditions must rely, at least in part, on self report
measures. Many lay persons, as well as beginning pain researchers
and clinicians, are uncomfortable with this reliance on self report.
"Isn't everyone's pain different?" "Pain seems
so subjective!" and "How can you study something so
unreliable?" are comments frequently heard. Despite these
misgivings, texts on the measurement of pain (e.g. Turk
and Melzack, 2001) indicate that certain self report measures,
including simple verbal descriptor scales and numerical rating
scales, have acceptable degrees of reliability (reproducibility)
and validity (i.e. measure what they are intended to measure).
Researchers
can enhance the reliability of pain measurement by increasing
the clarity and specificity of the questions asked. This is accomplished
by using standardized questions that are easily understood by
people of all educational levels and that are highly specific
in terms of pain location, pain severity, frequency, and duration
of pain, and the time period covered by the question. In addition,
in arriving at case definitions, self report measures may be supplemented
by data on behavior (e.g. work disability) and clinical signs.
But by definition, pain is a subjective experience. At this time,
the most valid method of assessing pain is by self report.
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