IDL Reference Guide: Procedures and Functions

CHISQR_PDF

The CHISQR_PDF function computes the probability P that, in a chi-square distribution with Df degrees of freedom, a random variable X is less than or equal to a user-specified cutoff value V.

This routine is written in the IDL language. Its source code can be found in the file chisqr_pdf.pro in the lib subdirectory of the IDL distribution.

Syntax

Result = CHISQR_PDF(V, Df)

Return Value

If both arguments are scalar, the function returns a scalar. If both arguments are arrays, the function matches up the corresponding elements of V and Df, returning an array with the same dimensions as the smallest array. If one argument is a scalar and the other argument is an array, the function uses the scalar value with each element of the array, and returns an array with the same dimensions as the input array.

If any of the arguments are double-precision, the result is double-precision, otherwise the result is single-precision.

Arguments

V

A scalar or array that specifies the cutoff value(s).

Df

A positive scalar or array that specifies the number of degrees of freedom of the chi-square distribution.

Keywords

None.

Examples

Use the following command to compute the probability that a random variable X, from the chi-square distribution with three degrees of freedom, is less than or equal to 6.25. The result should be 0.899939.

result = CHISQR_PDF(6.25, 3)  
PRINT, result  

IDL prints:

0.899939  

Compute the probability that a random variable X from the chi-square distribution with three degrees of freedom, is greater than 6.25. The result should be 0.100061.

PRINT, 1 - chisqr_pdf(6.25, 3)  

IDL prints:

0.100061