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Standard Normal Distribution Script Webb  all rights reserved  ©2004

Author  Russ Lenth
Email: russelllenth@stat.uiowa.edu
Webpage:
Summary: pdf, cdf, and quantile of the N(0,1) distribution.
Instructions: This adds basic functionality for the normal distribution.
The pdf at z is the function phi(z) = exp(.5 z^2) / sqrt(2pi).
The cdf is the function Phi(z) = integral phi(t)dt from t = infinity to t = z.
The quantile is the solution z to the equation Phi(z) = p where p is the value supplied.
The algorithm for the cdf is (26.2.16) in Abramowitz and Stegun, which is accurate to +/ 1e5. (It'd be easy but bulkier to use (26.2.17), accurate to +/ 1e8).
The quantile is obtained using the Tukey's lambda approximation z = 4.92{p^.14  (1p)^.14} as a starter, and doing one Newton step.
Code:
RPN.1
[d]g1*Hne#'.39894228'*;
[p]g10<(nCpn1+:g1#'.33267'*1+tg1g1#'.9372980'*#'.1201676'*#'.4361836'+*r2Cd*n1+);
[q]g1g1#'.14'Pr2n1+#'.14'P#'4.92'*r2g2Cpg2Cd/+;
"Normal dist"
"_p(x): pdf"Cd;
"_P(x): cdf"Cp;
"_Q(x): quantile"Cq;

