GraphPad Statistics Guide

How Prism computes exact P values

How Prism computes exact P values

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How Prism computes exact P values

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Calcuations built-in to Prism

GraphPad Prism report exact P values with most statistical calculations using these algorithms, adapted from sections 6.2 and 6.4 of Numerical Recipes.

PFromF(F_Ratio, DF_Numerator, DF_Denominator) =

BetaI(DF_Denominato /2, DF_Numerator/2, DF_Denominator / (DF_Denominator + DF_Numerator * F_Ratio))

 

PFromT(T_Ratio, DF) = BetaI(DF /2, 1/2, DF / (DF + T_Ratio^2))

 

PFromZ(Z_Ratio) = PFromT(Z_Ratio, Infinity)

 

PFromR(R_Value) = PFromT(|R_Value| / SQRT((1 - R_Value^2)/DF) , DF)

 

PFromChi2(Chi2_Value, DF) = GammaQ(DF / 2, Chi2Value /2)

Note that BetaI is the incomplete beta function, and GammaQ is the incomplete gamma function. The variable names should all be self-explanatory.

Calculations with newer versions of Excel

If you want to compute P values using newer (2010 and later) Excel, use these functions:

P value from F

=F.DIST.RT (F, DFn, DFd)

P value from t (two tailed)

=T.DIST.2T(t, df)

 

P value from Chi Square

=CHISQ.DIST.RT(ChiSquare, DF)

P value from z (two tailed)

=2*(1.0-NORM.S.DIST(z,TRUE))

Calculations with older versions of Excel

If you want to compute P values using older (pre 2010) Excel, use these functions:

P value from F

=FDIST (F, DFn, DFd)

P value from t (two tailed)

=TDIST (t, df, 2)
(The third argument, 2, specifies a two-tail P value.)

P value from Chi Square

=CHIDIST (ChiSquare, DF)

P value from z (two tailed)

=2*(1.0-NORMSDIST(z))

 

Reference

Numerical Recipes 3rd Edition: The Art of Scientific Computing, by William H. Press, Saul A. Teukolsky, William T. Vetterling, IBSN:0521880688.