Biostatistics For Dummies
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For a correlation test in biostatistics (such as the Pearson or Spearman test), pick the scatter chart that looks like an important amount of correlation. Each chart shows the value of r (the correlation coefficient) and the required number of analyzable subjects (each providing an x and a y value). For example, if the scatter chart in the lower left corner (corresponding to r = 0.6) appears to show an important amount of correlation, you’ll need about 20 analyzable subjects.

[Credit: Illustration by Wiley, Composition Services Graphics]
Credit: Illustration by Wiley, Composition Services Graphics

For other r values that aren’t in the preceding scatter charts, use this rule of thumb to estimate sample size: You need about 8/r2 – 3 analyzable subjects.

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John C. Pezzullo, PhD, has held faculty appointments in the departments of biomathematics and biostatistics, pharmacology, nursing, and internal medicine at Georgetown University. He is semi-retired and continues to teach biostatistics and clinical trial design online to Georgetown University students.

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