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A cross-sectional
study (also called a prevalence survey) measures the prevalence
of a symptom, determinants of a symptom, or both, in a population
at one point in time or over a short period of time. It provides
a snapshot of the health experience of a population at a given
time. Such information can be very useful in assessing the health
status and needs of a population. It can also be used to study
the relationship between variables (for example between breathlessness
and lung metastasis). The prevalence of a problem, rather than
the incidence, is recorded in a cross-sectional survey, and every
association should be interpreted cautiously. Bias may arise because
of selection into or out of the study population. For example,
in a hospital survey, patients staying for a shorter period in
hospital have less probability of being included in a cross-sectional
survey.
Figure 8.1 shows five patients over 6 days. The
lines represent time spent in the hospital. For example, patient
1 entered hospital on day 4. A cross sectional survey takes place
at day 3.
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8.1: Time Spent in a Hospital
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Question
8.1 Three of the five patients would be included in the
survey because patient #3 was discharged prior to the study start.
Question 8.2 If the survey were collected on
day 5, or day 2, four patients would be included on either occasion.
Question 8.3 Three of the patients would be the
same on either day 2 or day 5, but one patient would be different
on each day.
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