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Validity is the extent to which a measurement or study
reaches the correct conclusion.
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Bias is a systematic error that occurs in a study and leads
to a distorted result.
- Selection bias is a distortion in the results of a study that
arises because of the manner in which the subjects are sampled.
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Information (or misclassification) bias is a distortion in
the results of a study that arises because of the manner in which
data are collected or classified.
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When errors in classification of exposure or disease status
do not depend on the other variable, any bias is referred to as
nondifferential, and tends to underestimate the association.
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When errors in classification of exposure or disease status
depend upon the other variable, any bias is referred to as differential,
and may lead to over- or underestimating the association.
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Confounding is the mixing of the influence of an extraneous
factor with the effect of the exposure of interest.
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A 45-year-old man began working as a production supervisor, and
his employer required that he undergo a complete medical examination.
His physician learned that the patient’s father had died of
myocardial infarction at age 65. On physical examination, the patient
was moderately obese, and his blood pressure was 140/86.
The remainder of the examination revealed no notable abnormalities.
The patient’s total serum cholesterol level (nonfasting)
was 242 mg/dL.
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According to the guidelines of the National Cholesterol Education
Program (NCEP), a total serum cholesterol concentration greater
than 240 mg/dL is an indication for possible pharmacologic lowering
of serum cholesterol. A value of 200–239 mg/dL
is considered borderline and should trigger dietary intervention,
and a value less than 200 mg/dL is considered normal.
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Based on the initial cholesterol results, the physician asked
the patient to return in 2 weeks for further testing. On repeat
measurement, the total serum cholesterol concentration was 198 mg/dL
on a fasting lipid profile. Table 10–1 lists several different
factors that could explain the observed variability in measured
total serum cholesterol level. The source of this variability in
the measured total cholesterol level had important implications
for how the physician treated this patient.
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Difficulties in the interpretation of test results of individual
patients are magnified when groups of patients are studied. The
sources of variability in test results and errors in medical research
are discussed in this chapter. Appreciation of these issues is important
for the interpretation and appropriate application of research findings
in the clinical setting.
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Variability in measurements can be either random or systematic.
A schematic representation of random and systematic variation is
shown in Figure 10–1. The shots at the ...