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  • Image not available. Validity is the extent to which a measurement or study reaches the correct conclusion.
  • Image not available. Bias is a systematic error that occurs in a study and leads to a distorted result.
  • Image not available.Selection bias is a distortion in the results of a study that arises because of the manner in which the subjects are sampled.
  • Image not available. 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.
  • Image not available. 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.
  • Image not available. 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.
  • Image not available. Confounding is the mixing of the influence of an extraneous factor with the effect of the exposure of interest.


Image not available. 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.


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.


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.

Table Graphic Jump Location
Table 10–1. Levels of Variability.

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.


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 ...

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