Evaluates the noncentral Student's t cumulative probability distribution function.

Namespace: Imsl.Stat
Assembly: ImslCS (in ImslCS.dll) Version: 6.5.0.0

Syntax

C#
public static double NoncentralstudentsT(
	double t,
	int idf,
	double delta
)
Visual Basic (Declaration)
Public Shared Function NoncentralstudentsT ( _
	t As Double, _
	idf As Integer, _
	delta As Double _
) As Double
Visual C++
public:
static double NoncentralstudentsT(
	double t, 
	int idf, 
	double delta
)

Parameters

t
Type: System..::.Double
A double scalar value representing the argument at which the function is to be evaluated.
idf
Type: System..::.Int32
An int scalar value representing the number of degrees of freedom. This must be positive.
delta
Type: System..::.Double
A double scalar value representing the noncentrality parameter.

Return Value

A double scalar value representing the probability that a noncentral Student's t random variable takes a value less than or equal to t.

Remarks

Method Cdf.NoncentralstudentsT evaluates the distribution function F of a noncentral t random variable with idf degrees of freedom and noncentrality parameter delta; that is, with \nu=\mbox{idf} , \delta=\mbox{delta}, and t_{0}=\mbox{t},
F{\left({t_0}\right)}=\int_{-{\infty}}^{ t_{0}}{\frac{\nu^{\nu/2}e^{{-\delta^2}/2}}{{\sqrt{\pi}\Gamma\left( \nu/2\right)\left(\nu+x^2\right)}^{\left(\nu+1\right)/2}}}\sum \limits_{i=0}^\infty{\Gamma\left(\left(\nu+i+1\right)/2\right) \left(\frac{\delta^i}{i!}\right)\left(\frac{2x^2}{\nu+x^2}\right)^{ i/2}dx}
where \Gamma(\cdot) is the gamma function. The value of the distribution function at the point t_{0} is the probability that the random variable takes a value less than or equal to t_{0}.

The noncentral t random variable can be defined by the distribution function above, or alternatively and equivalently, as the ratio of a normal random variable and an independent chi-squared random variable. If w has a normal distribution with mean \delta and variance equal to one, u has an independent chi-squared distribution with \nu degrees of freedom, and

x = w/\sqrt{u/\nu}
then x has a noncentral t distribution with \nu degrees of freedom and noncentrality parameter \delta.

The distribution function of the noncentral t can also be expressed as a double integral involving a normal density function (see, for example, Owen 1962, page 108). The method Cdf.NoncentralstudentsT uses the method of Owen (1962, 1965), which uses repeated integration by parts on that alternate expression for the distribution function.

Noncentral Student's t Distribution Function

See Also