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betainc

Evaluates the incomplete beta function.

Prototype

	function betainc (
		x  : numeric,  
		a  : numeric,  
		b  : numeric   
	)

	return_val [dimsizes(x)] :  typeof(x)

Arguments

x

upper limit of integration. x may be of any dimensionality. x must be in (0,1) inclusive and can only be float or double.

a

first beta distribution parameter; must be > 0.0. It must be the same dimensionality as x.

b

second beta distribution parameter; must be > 0.0. It must be the same dimensionality as x.

Return value

The variable returned will be the same type and dimensionality as x.

As of NCL version 4.3.1, if x contains missing values, the return value will contain missing values in the same locations.

Description

betainc calculates the incomplete beta function. The incomplete beta function ratio is the probability that a random variable from a beta distribution having parameters a and b will be less than or equal to x. The code used is from SLATEC (http://www.netlib.org/slatec/fnlib/). This returns the same answers as the Numerical Recipes [Cambridge Univ. Press, 1986] function betai.

This function is often used to determine probabilities.

Note: in NCL version 4.3.1, this function was updated to handle missing values. If any missing values are inputted, the output array will contain missing values in the same locations.

Examples

Example 1

 
  a = 0.5
  b = 5.0
  x = 0.2

  alpha = betainc(x,a,b) 
  print("alpha(x,a,b)="+alpha)

  x = 0.5
  alpha = betainc(x,a,b) 
  print("alpha(x,a,b)="+alpha)

The result is:

  alpha(x,a,b)= 0.85507
  alpha(x,a,b)= 0.98988

Example 2 - The betainc can be used as a p-Value calculator for the Student t-test. Let's say a calculation has been made where the degrees-of-freedom (df=20) and a Student-t value of 2.08 has been determined. A probability level may be determined via:

  df   = 20 
  tval = 2.08  
  prob = betainc( df/(df+tval^2), df/2.0, 0.5)
  print ("prob="+prob)

The result is prob = 0.0506. This is a two-tailed probability. The one-tailed probability is 0.5*prob = 0.0253,

For plotting, users often prefer to plot the quantity:

   prob = (1.-betainc(x,a,b))*100.  ; probability in %

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