﻿ g01gd Method
g01gd returns the probability associated with the lower tail of the noncentral $F$ or variance-ratio distribution.

# Syntax

C#
```public static double g01gd(
double f,
double df1,
double df2,
double rlamda,
double tol,
int maxit,
out int ifail
)```
Visual Basic
```Public Shared Function g01gd ( _
f As Double, _
df1 As Double, _
df2 As Double, _
rlamda As Double, _
tol As Double, _
maxit As Integer, _
<OutAttribute> ByRef ifail As Integer _
) As Double```
Visual C++
```public:
static double g01gd(
double f,
double df1,
double df2,
double rlamda,
double tol,
int maxit,
[OutAttribute] int% ifail
)```
F#
```static member g01gd :
f : float *
df1 : float *
df2 : float *
rlamda : float *
tol : float *
maxit : int *
ifail : int byref -> float
```

#### Parameters

f
Type: System..::..Double
On entry: $f$, the deviate from the noncentral $F$-distribution.
Constraint: ${\mathbf{f}}>0.0$.
df1
Type: System..::..Double
On entry: the degrees of freedom of the numerator variance, ${\nu }_{1}$.
Constraint: $0.0<{\mathbf{df1}}\le {10}^{6}$.
df2
Type: System..::..Double
On entry: the degrees of freedom of the denominator variance, ${\nu }_{2}$.
Constraint: ${\mathbf{df2}}>0.0$.
rlamda
Type: System..::..Double
On entry: $\lambda$, the noncentrality parameter.
Constraint: $0.0\le {\mathbf{rlamda}}\le -2.0\mathrm{log}\left(U\right)$ where $U$ is the safe range parameter as defined by x02am.
tol
Type: System..::..Double
On entry: the relative accuracy required by you in the results. If g01gd is entered with tol greater than or equal to $1.0$ or less than  (see x02aj), then the value of  is used instead.
maxit
Type: System..::..Int32
On entry: the maximum number of iterations to be used.
Suggested value: $500$. See g01gc and g01ge for further details.
Constraint: ${\mathbf{maxit}}\ge 1$.
ifail
Type: System..::..Int32%
On exit: ${\mathbf{ifail}}={0}$ unless the method detects an error or a warning has been flagged (see [Error Indicators and Warnings]).

#### Return Value

g01gd returns the probability associated with the lower tail of the noncentral $F$ or variance-ratio distribution.

# Description

The lower tail probability of the noncentral $F$-distribution with ${\nu }_{1}$ and ${\nu }_{2}$ degrees of freedom and noncentrality parameter $\lambda$, $P\left(F\le f:{\nu }_{1},{\nu }_{2}\text{;}\lambda \right)$, is defined by
 $PF≤f:ν1,ν2;λ=∫0xpF:ν1,ν2;λdF,$
where
 $PF:ν1,ν2;λ=∑j=0∞e-λ/2λ/2jj!×ν1+2jν1+2j/2ν2ν2/2Bν1+2j/2,ν2/2$
 $×uν1+2j-2/2ν2+ν1+2ju-ν1+2j+ν2/2$
and $B\left(·,·\right)$ is the beta function.
The probability is computed by means of a transformation to a noncentral beta distribution:
 $PF≤f:ν1,ν2;λ=PβX≤x:a,b;λ,$
where $x=\frac{{\nu }_{1}f}{{\nu }_{1}f+{\nu }_{2}}$ and ${P}_{\beta }\left(X\le x:a,b\text{;}\lambda \right)$ is the lower tail probability integral of the noncentral beta distribution with parameters $a$, $b$, and $\lambda$.
If ${\nu }_{2}$ is very large, greater than ${10}^{6}$, then a ${\chi }^{2}$ approximation is used.

# References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications

# Error Indicators and Warnings

Note: g01gd may return useful information for one or more of the following detected errors or warnings.
Errors or warnings detected by the method:
If on exit ${\mathbf{ifail}}={1}$ or ${3}$, then g01gd returns $0.0$.
${\mathbf{ifail}}=1$
 On entry, ${\mathbf{df1}}\le 0.0$, or ${\mathbf{df1}}>{10}^{6}$, or ${\mathbf{df2}}\le 0.0$, or ${\mathbf{f}}\le 0.0$, or ${\mathbf{rlamda}}<0.0$, or ${\mathbf{maxit}}<1$, or ${\mathbf{rlamda}}>-2.0\mathrm{log}\left(U\right)$, where $U=\text{}$ safe range parameter as defined by x02am.
${\mathbf{ifail}}=2$
The solution has failed to converge in maxit iterations. You should try a larger value of maxit or tol.
${\mathbf{ifail}}=3$
The required probability cannot be computed accurately. This may happen if the result would be very close to $0.0$ or $1.0$. Alternatively the values of df1 and f may be too large. In the latter case you could try using a normal approximation; see Abramowitz and Stegun (1972).
${\mathbf{ifail}}=4$
The required accuracy was not achieved when calculating the initial value of the central $F$ (or ${\chi }^{2}$) probability. You should try a larger value of tol. If the ${\chi }^{2}$ approximation is being used then g01gd returns zero otherwise the value returned should be an approximation to the correct value.
${\mathbf{ifail}}=-9000$
An error occured, see message report.

# Accuracy

The relative accuracy should be as specified by tol. For further details see g01gc and g01ge.

# Parallelism and Performance

None.

When both ${\nu }_{1}$ and ${\nu }_{2}$ are large a Normal approximation may be used and when only ${\nu }_{1}$ is large a ${\chi }^{2}$ approximation may be used. In both cases $\lambda$ is required to be of the same order as ${\nu }_{1}$. See Abramowitz and Stegun (1972) for further details.

# Example

This example reads values from, and degrees of freedom for, $F$-distributions, computes the lower tail probabilities and prints all these values until the end of data is reached.

Example program (C#): g01gde.cs

Example program data: g01gde.d

Example program results: g01gde.r