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Biostatistics & Research · single question

Specificity is calculated as:

  1. TP / (TP + FP) — positive predictive value
  2. TN / (TN + FP) — true negative rate
  3. TN / (TN + FN) — negative predictive value
  4. TP / (TP + FN) — true positive rate (sensitivity)
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Correct answer: B. TN / (TN + FP) — true negative rate

  • A. TP / (TP + FP) = Positive Predictive Value (PPV) — probability the test is correct when positive.
  • B. Correct. Specificity = TN / (TN + FP). It measures the proportion of truly disease-FREE individuals correctly identified by a NEGATIVE test result. High specificity = few false positives ('rule in' disease). Mnemonic: 'SpPin' — a Specific test, when Positive, rules In disease.
  • C. TN / (TN + FN) = Negative Predictive Value (NPV) — probability the test is correct when negative.
  • D. TP / (TP + FN) = Sensitivity (true positive rate) — identifies those WITH disease. Mnemonic: 'SnNout' — Sensitive test, when Negative, rules Out disease.

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