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Statistical Calculations

Diagnostic Tests

Calculators

Centre for Evidence Based Medicine Stats Calculator

Vanderbilt- Power and Sample Size Calculator

Calculations

 Gold standard Positive (condition present) Gold standard Negative (condition not present) Test result Positive True Positive False Positive Test result Negative False Negative True Negative

Stable Properties:

Sensitivity = True Positives/(True Positives + False Negatives)

Specificity = True Negatives/(False Positive + True Negative)

Frequency Dependent Properties:

Positive Predictive Value = True Positive/(True Positive + False Positive)

Negative Predictive Value = True Negative/(True Negative + False Negative)

Likelihood Ratios

The likelihood ratio for a test result compares the likelihood of that result in patients with disease to the likelihood of that result in patients without disease:

 Condition Present Condition Absent Test Positive a b Test Negative c d

Positive LR = (a/a+c)/(b/b+d)
Negative LR = (c/a+c)/(d/b+d)

How much do Likelihood Ratios change disease likelihood?

LRs >10 or <0.1 cause large changes in likelihood.
LRs 5-10 or 0.1-0.2 cause moderate changes.
LRs 2-5 or 0.2-0.5 cause small changes.
LRs between <2 and 0.5 cause little or no change.

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