3 * Inverse of Normal distribution function
9 * float x, y, ndtrif();
17 * Returns the argument, x, for which the area under the
18 * Gaussian probability density function (integrated from
19 * minus infinity to x) is equal to y.
22 * For small arguments 0 < y < exp(-2), the program computes
23 * z = sqrt( -2.0 * log(y) ); then the approximation is
24 * x = z - log(z)/z - (1/z) P(1/z) / Q(1/z).
25 * There are two rational functions P/Q, one for 0 < y < exp(-32)
26 * and the other for y up to exp(-2). For larger arguments,
27 * w = y - 0.5, and x/sqrt(2pi) = w + w**3 R(w**2)/S(w**2)).
33 * arithmetic domain # trials peak rms
34 * IEEE 1e-38, 1 30000 3.6e-7 5.0e-8
39 * message condition value returned
40 * ndtrif domain x <= 0 -MAXNUM
41 * ndtrif domain x >= 1 MAXNUM
47 Cephes Math Library Release 2.2: July, 1992
48 Copyright 1984, 1987, 1989, 1992 by Stephen L. Moshier
49 Direct inquiries to 30 Frost Street, Cambridge, MA 02140
56 static float s2pi = 2.50662827463100050242;
58 /* approximation for 0 <= |y - 0.5| <= 3/8 */
59 static float P0[5] = {
60 -5.99633501014107895267E1,
61 9.80010754185999661536E1,
62 -5.66762857469070293439E1,
63 1.39312609387279679503E1,
64 -1.23916583867381258016E0,
66 static float Q0[8] = {
67 /* 1.00000000000000000000E0,*/
68 1.95448858338141759834E0,
69 4.67627912898881538453E0,
70 8.63602421390890590575E1,
71 -2.25462687854119370527E2,
72 2.00260212380060660359E2,
73 -8.20372256168333339912E1,
74 1.59056225126211695515E1,
75 -1.18331621121330003142E0,
78 /* Approximation for interval z = sqrt(-2 log y ) between 2 and 8
79 * i.e., y between exp(-2) = .135 and exp(-32) = 1.27e-14.
81 static float P1[9] = {
82 4.05544892305962419923E0,
83 3.15251094599893866154E1,
84 5.71628192246421288162E1,
85 4.40805073893200834700E1,
86 1.46849561928858024014E1,
87 2.18663306850790267539E0,
88 -1.40256079171354495875E-1,
89 -3.50424626827848203418E-2,
90 -8.57456785154685413611E-4,
92 static float Q1[8] = {
93 /* 1.00000000000000000000E0,*/
94 1.57799883256466749731E1,
95 4.53907635128879210584E1,
96 4.13172038254672030440E1,
97 1.50425385692907503408E1,
98 2.50464946208309415979E0,
99 -1.42182922854787788574E-1,
100 -3.80806407691578277194E-2,
101 -9.33259480895457427372E-4,
105 /* Approximation for interval z = sqrt(-2 log y ) between 8 and 64
106 * i.e., y between exp(-32) = 1.27e-14 and exp(-2048) = 3.67e-890.
109 static float P2[9] = {
110 3.23774891776946035970E0,
111 6.91522889068984211695E0,
112 3.93881025292474443415E0,
113 1.33303460815807542389E0,
114 2.01485389549179081538E-1,
115 1.23716634817820021358E-2,
116 3.01581553508235416007E-4,
117 2.65806974686737550832E-6,
118 6.23974539184983293730E-9,
120 static float Q2[8] = {
121 /* 1.00000000000000000000E0,*/
122 6.02427039364742014255E0,
123 3.67983563856160859403E0,
124 1.37702099489081330271E0,
125 2.16236993594496635890E-1,
126 1.34204006088543189037E-2,
127 3.28014464682127739104E-4,
128 2.89247864745380683936E-6,
129 6.79019408009981274425E-9,
133 float polevlf(float, float *, int);
134 float p1evlf(float, float *, int);
135 float logf(float), sqrtf(float);
137 float polevlf(), p1evlf(), logf(), sqrtf();
142 float ndtrif(float yy0)
148 float y0, x, y, z, y2, x0, x1;
154 mtherr( "ndtrif", DOMAIN );
159 mtherr( "ndtrif", DOMAIN );
164 if( y > (1.0 - 0.13533528323661269189) ) /* 0.135... = exp(-2) */
170 if( y > 0.13533528323661269189 )
174 x = y + y * (y2 * polevlf( y2, P0, 4)/p1evlf( y2, Q0, 8 ));
179 x = sqrtf( -2.0 * logf(y) );
183 if( x < 8.0 ) /* y > exp(-32) = 1.2664165549e-14 */
184 x1 = z * polevlf( z, P1, 8 )/p1evlf( z, Q1, 8 );
186 x1 = z * polevlf( z, P2, 8 )/p1evlf( z, Q2, 8 );