NAG CL Interface
g01edc (prob_​f)

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1 Purpose

g01edc returns the probability for the lower or upper tail of the F or variance-ratio distribution with real degrees of freedom.

2 Specification

#include <nag.h>
double  g01edc (Nag_TailProbability tail, double f, double df1, double df2, NagError *fail)
The function may be called by the names: g01edc, nag_stat_prob_f or nag_prob_f_dist.

3 Description

The lower tail probability for the F, or variance-ratio distribution, with ν1 and ν2 degrees of freedom, P(Ff:ν1,ν2), is defined by:
P(Ff:ν1,ν2)=ν1ν1/2ν2ν2/2 Γ ((ν1+ν2)/2) Γ(ν1/2) Γ(ν2/2) 0fF(ν1-2)/2(ν1F+ν2)-(ν1+ν2)/2dF,  
for ν1, ν2>0, f0.
The probability is computed by means of a transformation to a beta distribution, Pβ(Bβ:a,b):
P(Ff:ν1,ν2)=Pβ (Bν1f ν1f+ν2 :ν1/2,ν2/2)  
and using a call to g01eec.
For very large values of both ν1 and ν2, greater than 105, a normal approximation is used. If only one of ν1 or ν2 is greater than 105 then a χ2 approximation is used, see Abramowitz and Stegun (1972).

4 References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
Hastings N A J and Peacock J B (1975) Statistical Distributions Butterworth

5 Arguments

1: tail Nag_TailProbability Input
On entry: indicates whether an upper or lower tail probability is required.
The lower tail probability is returned, i.e., P(Ff:ν1,ν2).
The upper tail probability is returned, i.e., P(Ff:ν1,ν2).
Constraint: tail=Nag_LowerTail or Nag_UpperTail.
2: f double Input
On entry: f, the value of the F variate.
Constraint: f0.0.
3: df1 double Input
On entry: the degrees of freedom of the numerator variance, ν1.
Constraint: df1>0.0.
4: df2 double Input
On entry: the degrees of freedom of the denominator variance, ν2.
Constraint: df2>0.0.
5: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

Note: on any of the error conditions listed below except fail.code= NE_PROBAB_CLOSE_TO_TAIL g01edc returns 0.0.
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument value had an illegal value.
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
The probability is too close to 0.0 or 1.0. f is too far out into the tails for the probability to be evaluated exactly. The result tends to approach 1.0 if f is large, or 0.0 if f is small. The result returned is a good approximation to the required solution.
On entry, df1=value and df2=value.
Constraint: df1>0.0 and df2>0.0.
On entry, f=value.
Constraint: f0.0.

7 Accuracy

The result should be accurate to five significant digits.

8 Parallelism and Performance

g01edc is not threaded in any implementation.

9 Further Comments

For higher accuracy g01eec can be used along with the transformations given in Section 3.

10 Example

This example reads values from, and degrees of freedom for, a number of F-distributions and computes the associated lower tail probabilities.

10.1 Program Text

Program Text (g01edce.c)

10.2 Program Data

Program Data (g01edce.d)

10.3 Program Results

Program Results (g01edce.r)