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Studies in which the investigator passively observes events
without determining assignment of exposure are referred to as observational.- Cohort studies are observational studies in which the investigator
determines the exposure status of subjects and then follows them
for subsequent outcomes.
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A prospective cohort study is one in which the exposure and
subsequent outcome status of each subject is determined after the
onset of the investigation.
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A retrospective cohort study is one which utilizes historical
information on exposure status and subsequent outcome.
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The risk ratio or relative risk is a measure of association
between the exposure and disease and is defined as the risk among
the exposed divided by the risk among the unexposed.
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A risk ratio > 1 implies that exposure increases the risk
of disease, whereas a risk ratio < 1 implies that exposure decreases
the risk of disease.
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The attributable risk percentage is a measure of the proportion
of the total risk among exposed persons that is related to their
exposure.
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A pediatrician was called to the hospital to attend the delivery
of a newborn. The mother, a 28-year-old primigravida, had experienced
elevated blood pressure during an otherwise uncomplicated pregnancy.
The labor was induced because the pregnancy had continued 2 weeks
past the expected date of delivery. During labor, evidence of fetal
distress was seen. When the membranes ruptured, the obstetrician
noted thick greenish fluid containing meconium. At the time of delivery, the
male newborn was limp and cyanotic, and had no spontaneous respiratory
effort and a heart rate of only 50 beats/minute. When meconium
was suctioned from his mouth and nose, the baby did not grimace,
cough, or sneeze.
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Vigorous efforts at resuscitation were initiated, including bag-and-mask
ventilation with 100% oxygen and chest compressions, but
the Apgar score at 1 minute of life was 1. The Apgar score (Table
8–1), an index of neonatal asphyxia, can range from 0 (very
asphyxiated) to 10 (no asphyxia). Despite continuing resuscitation,
the 5-minute Apgar score improved only to 2, with a heart rate of
110 beats/minute. The 10-minute Apgar score remained depressed
at 3, and the neonate was transferred to the newborn intensive care
unit. With aggressive medical management, the 3100-g neonate continued
to improve without evidence of acute neurologic complications. He was
discharged from the hospital on the twelfth day of life.
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