El CDF normal inverso, incluidos los coeficientes, se describe here. Y el valor absoluto del error relativo es inferior a 1,15 × 10-9
public static class NormalDistributionConfidenceCalculator
{
/// <summary>
///
/// </summary>
public static double InverseNormalDistribution(double probability, double min, double max)
{
double x = 0;
double a = 0;
double b = 1;
double precision = Math.Pow(10, -3);
while ((b - a) > precision)
{
x = (a + b)/2;
if (NormInv(x) > probability)
{
b = x;
}
else
{
a = x;
}
}
if ((max > 0) && (min > 0))
{
x = x * (max - min) + min;
}
return x;
}
/// <summary>
/// Returns the cumulative density function evaluated at A given value.
/// </summary>
/// <param name="x">A position on the x-axis.</param>
/// <param name="mean"></param>
/// <param name="sigma"></param>
/// <returns>The cumulative density function evaluated at <C>x</C>.</returns>
/// <remarks>The value of the cumulative density function at A point <C>x</C> is
/// probability that the value of A random variable having this normal density is
/// less than or equal to <C>x</C>.
/// </remarks>
public static double NormalDistribution(double x, double mean, double sigma)
{
// This algorithm is ported from dcdflib:
// Cody, W.D. (1993). "ALGORITHM 715: SPECFUN - A Portabel FORTRAN
// Package of Special Function Routines and Test Drivers"
// acm Transactions on Mathematical Software. 19, 22-32.
int i;
double del, xden, xnum, xsq;
double result, ccum;
double arg = (x - mean)/sigma;
const double sixten = 1.60e0;
const double sqrpi = 3.9894228040143267794e-1;
const double thrsh = 0.66291e0;
const double root32 = 5.656854248e0;
const double zero = 0.0e0;
const double min = Double.Epsilon;
double z = arg;
double y = Math.Abs(z);
const double half = 0.5e0;
const double one = 1.0e0;
double[] a =
{
2.2352520354606839287e00, 1.6102823106855587881e02, 1.0676894854603709582e03,
1.8154981253343561249e04, 6.5682337918207449113e-2
};
double[] b =
{
4.7202581904688241870e01, 9.7609855173777669322e02, 1.0260932208618978205e04,
4.5507789335026729956e04
};
double[] c =
{
3.9894151208813466764e-1, 8.8831497943883759412e00, 9.3506656132177855979e01,
5.9727027639480026226e02, 2.4945375852903726711e03, 6.8481904505362823326e03,
1.1602651437647350124e04, 9.8427148383839780218e03, 1.0765576773720192317e-8
};
double[] d =
{
2.2266688044328115691e01, 2.3538790178262499861e02, 1.5193775994075548050e03,
6.4855582982667607550e03, 1.8615571640885098091e04, 3.4900952721145977266e04,
3.8912003286093271411e04, 1.9685429676859990727e04
};
double[] p =
{
2.1589853405795699e-1, 1.274011611602473639e-1, 2.2235277870649807e-2,
1.421619193227893466e-3, 2.9112874951168792e-5, 2.307344176494017303e-2
};
double[] q =
{
1.28426009614491121e00, 4.68238212480865118e-1, 6.59881378689285515e-2,
3.78239633202758244e-3, 7.29751555083966205e-5
};
if (y <= thrsh)
{
//
// Evaluate anorm for |X| <= 0.66291
//
xsq = zero;
if (y > double.Epsilon) xsq = z * z;
xnum = a[4] * xsq;
xden = xsq;
for (i = 0; i < 3; i++)
{
xnum = (xnum + a[i]) * xsq;
xden = (xden + b[i]) * xsq;
}
result = z * (xnum + a[3])/(xden + b[3]);
double temp = result;
result = half + temp;
}
//
// Evaluate anorm for 0.66291 <= |X| <= sqrt(32)
//
else if (y <= root32)
{
xnum = c[8] * y;
xden = y;
for (i = 0; i < 7; i++)
{
xnum = (xnum + c[i]) * y;
xden = (xden + d[i]) * y;
}
result = (xnum + c[7])/(xden + d[7]);
xsq = Math.Floor(y * sixten)/sixten;
del = (y - xsq) * (y + xsq);
result = Math.Exp(-(xsq * xsq * half)) * Math.Exp(-(del * half)) * result;
ccum = one - result;
if (z > zero)
{
result = ccum;
}
}
//
// Evaluate anorm for |X| > sqrt(32)
//
else
{
xsq = one/(z * z);
xnum = p[5] * xsq;
xden = xsq;
for (i = 0; i < 4; i++)
{
xnum = (xnum + p[i]) * xsq;
xden = (xden + q[i]) * xsq;
}
result = xsq * (xnum + p[4])/(xden + q[4]);
result = (sqrpi - result)/y;
xsq = Math.Floor(z * sixten)/sixten;
del = (z - xsq) * (z + xsq);
result = Math.Exp(-(xsq * xsq * half)) * Math.Exp(-(del * half)) * result;
ccum = one - result;
if (z > zero)
{
result = ccum;
}
}
if (result < min)
result = 0.0e0;
return result;
}
/// <summary>
/// Given a probability, a mean, and a standard deviation, an x value can be calculated.
/// </summary>
/// <returns></returns>
public static double NormInv(double probability)
{
const double a1 = -39.6968302866538;
const double a2 = 220.946098424521;
const double a3 = -275.928510446969;
const double a4 = 138.357751867269;
const double a5 = -30.6647980661472;
const double a6 = 2.50662827745924;
const double b1 = -54.4760987982241;
const double b2 = 161.585836858041;
const double b3 = -155.698979859887;
const double b4 = 66.8013118877197;
const double b5 = -13.2806815528857;
const double c1 = -7.78489400243029E-03;
const double c2 = -0.322396458041136;
const double c3 = -2.40075827716184;
const double c4 = -2.54973253934373;
const double c5 = 4.37466414146497;
const double c6 = 2.93816398269878;
const double d1 = 7.78469570904146E-03;
const double d2 = 0.32246712907004;
const double d3 = 2.445134137143;
const double d4 = 3.75440866190742;
//Define break-points
// using Epsilon is wrong; see link above for reference to 0.02425 value
//const double pLow = double.Epsilon;
const double pLow = 0.02425;
const double pHigh = 1 - pLow;
//Define work variables
double q;
double result = 0;
// if argument out of bounds.
// set it to a value within desired precision.
if (probability <= 0)
probability = pLow;
if (probability >= 1)
probability = pHigh;
if (probability < pLow)
{
//Rational approximation for lower region
q = Math.Sqrt(-2 * Math.Log(probability));
result = (((((c1 * q + c2) * q + c3) * q + c4) * q + c5) * q + c6)/((((d1 * q + d2) * q + d3) * q + d4) * q + 1);
}
else if (probability <= pHigh)
{
//Rational approximation for lower region
q = probability - 0.5;
double r = q * q;
result = (((((a1 * r + a2) * r + a3) * r + a4) * r + a5) * r + a6) * q/
(((((b1 * r + b2) * r + b3) * r + b4) * r + b5) * r + 1);
}
else if (probability < 1)
{
//Rational approximation for upper region
q = Math.Sqrt(-2 * Math.Log(1 - probability));
result = -(((((c1 * q + c2) * q + c3) * q + c4) * q + c5) * q + c6)/((((d1 * q + d2) * q + d3) * q + d4) * q + 1);
}
return result;
}
/// <summary>
///
/// </summary>
/// <param name="probability"></param>
/// <param name="mean"></param>
/// <param name="sigma"></param>
/// <returns></returns>
public static double NormInv(double probability, double mean, double sigma)
{
double x = NormInv(probability);
return sigma * x + mean;
}
}
Haga lo que haga, * no * instale Excel en su servidor web. Escribe tu propia función para esto si es necesario. – MusiGenesis
@MusiGenesis: ¿Por qué eres tan hostil a Excel en un servidor web? Estoy de acuerdo en que no es * ideal *, pero parece mejor que tratar de escribir mis propias funciones estadísticas, ¿verdad? – Portman
No soy hostil a Excel en un servidor web; estoy mortalmente aterrorizado. Los objetos de Office son * cosas * enormes, y no desea que su aplicación web tenga el impacto de instanciarlos para algo tan pequeño como una función de distribución. Es cierto que no quieres reinventar la rueda, pero también es cierto que no quieres matar ratones con obuses. Definitivamente es mejor que escribas tu propia función. – MusiGenesis