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

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/*r*^{2} – 3 analyzable subjects.