function phi=stdnormcdf(z)
% stdnormcdf(z) calculates the value of the standard normal 
% cumulative distribution function at z, 
% using the built-in MATLAB error function erf;
% see Abramowitz and Stegun, 1964, Handbook of Mathematical Functions,
% Dover, New York, equation 7.1.22
phi=0.5*(1 + (erf(z/sqrt(2))));