Statistical considerations for a study of the effect of one variable on another

Note: The power calculation uses the non-central t function, pt(x,df,ncen), and it's inverse qt
Power=pt(qt(.025,n-1,0),n-1,-(delta/sigma)*sqrt(n)), power is truncated rather than rounded.
If power is specified the other parameters are found by searching.


Significance Level — sided (default is 0.05, two-sided)

Standard deviation of the dependent variable (if known)

Standard deviation of the independent variable (if known)

Enter two of the following three values and the remaining value will be calculated

  1. Total number of patients
  2. Power (usually 0.8 or 0.9)
  3. Minimal detectable difference (change in the dependent variable per unit change in the independent variable; if the standard deviations (above) are not known, the units are "standard deviations")

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This software developed by David Schoenfeld, Ph.D. (, with support from the MGH Mallinckrodt General Clinical Research Center. Javascript version developed by REMorse.

These calculations are based on assumptions which may not be true for the clinical trial that you are planning. We do not guarantee the accuracy of these calculations or their suitability for your application. We suggest that you speak to a biostatistical consultant when planning a clinical trial. Please contact us if you have any questions or problems using this software