# Fd-class

##### Class "Fd"

The F distribution with `df1 =`

$n_1$, by default `= 1`

,
and `df2 =`

$n_2$, by default `= 1`

, degrees of freedom has density
$$d(x) = \frac{\Gamma(n_1/2 + n_2/2)}{\Gamma(n_1/2)\Gamma(n_2/2)}
\left(\frac{n_1}{n_2}\right)^{n_1/2} x^{n_1/2 -1}
\left(1 + \frac{n_1 x}{n_2}\right)^{-(n_1 + n_2) / 2}$$
for $x > 0$.
C.f. `rf`

- Keywords
- distribution

##### Note

It is the distribution of the ratio of the mean squares of n1 and n2 independent standard normals, and hence of the ratio of two independent chi-squared variates each divided by its degrees of freedom. Since the ratio of a normal and the root mean-square of m independent normals has a Student's $t_m$ distribution, the square of a $t_m$ variate has a F distribution on 1 and m degrees of freedom. The non-central F distribution is again the ratio of mean squares of independent normals of unit variance, but those in the numerator are allowed to have non-zero means and ncp is the sum of squares of the means.

##### Objects from the Class

Objects can be created by calls of the form `Fd(df1, df2)`

.
This object is a F distribution.

##### Extends

Class `"AbscontDistribution"`

, directly.
Class `"UnivariateDistribution"`

, by class `"AbscontDistribution"`

.
Class `"Distribution"`

, by class `"AbscontDistribution"`

.

##### Ad hoc methods

- An ad hoc method is provided for slot
`d`

if`ncp!=0`

. - For R Version
`<2.3.0< code="">ad hoc methods are provided for slots`

`q`

,`r`

if`ncp!=0`

; for R Version`>=2.3.0`

the methods from packagestats are used.

##### See Also

##### Examples

```
F <- Fd(df1=1,df2=1) # F is a F distribution with df=1 and df2=1.
r(F)(1) # one random number generated from this distribution, e.g. 29.37863
d(F)(1) # Density of this distribution is 0.1591549 for x=1 .
p(F)(1) # Probability that x<1 is 0.5.
q(F)(.1) # Probability that x<0.02508563 is 0.1.
df1(F) # df1 of this distribution is 1.
df1(F)=2 # df1 of this distribution is now 2.
Fn <- Fd(df1=1,df2=1,ncp=0.5)
# Fn is a F distribution with df=1, df2=1 and ncp =0.5.
d(Fn)(1) ## from R 2.3.0 on ncp no longer ignored...
```

*Documentation reproduced from package distr, version 1.8, License: GPL (version 2 or later)*