JavaScript library for calculating critical values and upper probabilities of common statistical distributions

statistics-distributions.js

/*
 * NAME
 * 
 * statistics-distributions.js - JavaScript library for calculating
 *   critical values and upper probabilities of common statistical
 *   distributions
 * 
 * SYNOPSIS
 * 
 * 
 *   // Chi-squared-crit (2 degrees of freedom, 95th percentile = 0.05 level
 *   chisqrdistr(2, .05)
 *   
 *   // u-crit (95th percentile = 0.05 level)
 *   udistr(.05);
 *   
 *   // t-crit (1 degree of freedom, 99.5th percentile = 0.005 level) 
 *   tdistr(1,.005);
 *   
 *   // F-crit (1 degree of freedom in numerator, 3 degrees of freedom 
 *   //         in denominator, 99th percentile = 0.01 level)
 *   fdistr(1,3,.01);
 *   
 *   // upper probability of the u distribution (u = -0.85): Q(u) = 1-G(u)
 *   uprob(-0.85);
 *   
 *   // upper probability of the chi-square distribution
 *   // (3 degrees of freedom, chi-squared = 6.25): Q = 1-G
 *   chisqrprob(3,6.25);
 *   
 *   // upper probability of the t distribution
 *   // (3 degrees of freedom, t = 6.251): Q = 1-G
 *   tprob(3,6.251);
 *   
 *   // upper probability of the F distribution
 *   // (3 degrees of freedom in numerator, 5 degrees of freedom in
 *   //  denominator, F = 6.25): Q = 1-G
 *   fprob(3,5,.625);
 * 
 * 
 *  DESCRIPTION
 * 
 * This library calculates percentage points (5 significant digits) of the u
 * (standard normal) distribution, the student's t distribution, the
 * chi-square distribution and the F distribution. It can also calculate the
 * upper probability (5 significant digits) of the u (standard normal), the
 * chi-square, the t and the F distribution.
 * 
 * These critical values are needed to perform statistical tests, like the u
 * test, the t test, the F test and the chi-squared test, and to calculate
 * confidence intervals.
 * 
 * If you are interested in more precise algorithms you could look at:
 *   StatLib: http://lib.stat.cmu.edu/apstat/ ; 
 *   Applied Statistics Algorithms by Griffiths, P. and Hill, I.D.
 *   , Ellis Horwood: Chichester (1985)
 * 
 * BUGS 
 * 
 * This port was produced from the Perl module Statistics::Distributions
 * that has had no bug reports in several years.  If you find a bug then
 * please double-check that JavaScript does not thing the numbers you are
 * passing in are strings.  (You can subtract 0 from them as you pass them
 * in so that "5" is properly understood to be 5.)  If you have passed in a
 * number then please contact the author
 * 
 * AUTHOR
 * 
 * Ben Tilly <[email protected]>
 * 
 * Originl Perl version by Michael Kospach <[email protected]>
 * 
 * Nice formating, simplification and bug repair by Matthias Trautner Kromann
 * <[email protected]>
 * 
 * COPYRIGHT 
 * 
 * Copyright 2008 Ben Tilly.
 * 
 * This library is free software; you can redistribute it and/or modify it
 * under the same terms as Perl itself.  This means under either the Perl
 * Artistic License or the GPL v1 or later.
 */

var SIGNIFICANT = 5; // number of significant digits to be returned

function chisqrdistr ($n, $p) {
	if ($n <= 0 || Math.abs($n) - Math.abs(integer($n)) != 0) {
		throw("Invalid n: $n\n"); /* degree of freedom */
	}
	if ($p <= 0 || $p > 1) {
		throw("Invalid p: $p\n"); 
	}
	return precision_string(_subchisqr($n-0, $p-0));
}

function udistr ($p) {
	if ($p > 1 || $p <= 0) {
		throw("Invalid p: $p\n");
	}
	return precision_string(_subu($p-0));
}

function tdistr ($n, $p) {
	if ($n <= 0 || Math.abs($n) - Math.abs(integer($n)) != 0) {
		throw("Invalid n: $n\n");
	}
	if ($p <= 0 || $p >= 1) {
		throw("Invalid p: $p\n");
	}
	return precision_string(_subt($n-0, $p-0));
}

function fdistr ($n, $m, $p) {
	if (($n<=0) || ((Math.abs($n)-(Math.abs(integer($n))))!=0)) {
		throw("Invalid n: $n\n"); /* first degree of freedom */
	}
	if (($m<=0) || ((Math.abs($m)-(Math.abs(integer($m))))!=0)) {
		throw("Invalid m: $m\n"); /* second degree of freedom */
	}
	if (($p<=0) || ($p>1)) {
		throw("Invalid p: $p\n");
	}
	return precision_string(_subf($n-0, $m-0, $p-0));
}

function uprob ($x) {
	return precision_string(_subuprob($x-0));
}

function chisqrprob ($n,$x) {
	if (($n <= 0) || ((Math.abs($n) - (Math.abs(integer($n)))) != 0)) {
		throw("Invalid n: $n\n"); /* degree of freedom */
	}
	return precision_string(_subchisqrprob($n-0, $x-0));
}

function tprob ($n, $x) {
	if (($n <= 0) || ((Math.abs($n) - Math.abs(integer($n))) !=0)) {
		throw("Invalid n: $n\n"); /* degree of freedom */
	}
	return precision_string(_subtprob($n-0, $x-0));
}

function fprob ($n, $m, $x) {
	if (($n<=0) || ((Math.abs($n)-(Math.abs(integer($n))))!=0)) {
		throw("Invalid n: $n\n"); /* first degree of freedom */
	}
	if (($m<=0) || ((Math.abs($m)-(Math.abs(integer($m))))!=0)) {
		throw("Invalid m: $m\n"); /* second degree of freedom */
	} 
	return precision_string(_subfprob($n-0, $m-0, $x-0));
}


function _subfprob ($n, $m, $x) {
	var $p;

	if ($x<=0) {
		$p=1;
	} else if ($m % 2 == 0) {
		var $z = $m / ($m + $n * $x);
		var $a = 1;
		for (var $i = $m - 2; $i >= 2; $i -= 2) {
			$a = 1 + ($n + $i - 2) / $i * $z * $a;
		}
		$p = 1 - Math.pow((1 - $z), ($n / 2) * $a);
	} else if ($n % 2 == 0) {
		var $z = $n * $x / ($m + $n * $x);
		var $a = 1;
		for (var $i = $n - 2; $i >= 2; $i -= 2) {
			$a = 1 + ($m + $i - 2) / $i * $z * $a;
		}
		$p = Math.pow((1 - $z), ($m / 2)) * $a;
	} else {
		var $y = Math.atan2(Math.sqrt($n * $x / $m), 1);
		var $z = Math.pow(Math.sin($y), 2);
		var $a = ($n == 1) ? 0 : 1;
		for (var $i = $n - 2; $i >= 3; $i -= 2) {
			$a = 1 + ($m + $i - 2) / $i * $z * $a;
		} 
		var $b = Math.PI;
		for (var $i = 2; $i <= $m - 1; $i += 2) {
			$b *= ($i - 1) / $i;
		}
		var $p1 = 2 / $b * Math.sin($y) * Math.pow(Math.cos($y), $m) * $a;

		$z = Math.pow(Math.cos($y), 2);
		$a = ($m == 1) ? 0 : 1;
		for (var $i = $m-2; $i >= 3; $i -= 2) {
			$a = 1 + ($i - 1) / $i * $z * $a;
		}
		$p = max(0, $p1 + 1 - 2 * $y / Math.PI
			- 2 / Math.PI * Math.sin($y) * Math.cos($y) * $a);
	}
	return $p;
}


function _subchisqrprob ($n,$x) {
	var $p;

	if ($x <= 0) {
		$p = 1;
	} else if ($n > 100) {
		$p = _subuprob((Math.pow(($x / $n), 1/3)
				- (1 - 2/9/$n)) / Math.sqrt(2/9/$n));
	} else if ($x > 400) {
		$p = 0;
	} else {   
		var $a;
                var $i;
                var $i1;
		if (($n % 2) != 0) {
			$p = 2 * _subuprob(Math.sqrt($x));
			$a = Math.sqrt(2/Math.PI) * Math.exp(-$x/2) / Math.sqrt($x);
			$i1 = 1;
		} else {
			$p = $a = Math.exp(-$x/2);
			$i1 = 2;
		}

		for ($i = $i1; $i <= ($n-2); $i += 2) {
			$a *= $x / $i;
			$p += $a;
		}
	}
	return $p;
}

function _subu ($p) {
	var $y = -Math.log(4 * $p * (1 - $p));
	var $x = Math.sqrt(
		$y * (1.570796288
		  + $y * (.03706987906
		  	+ $y * (-.8364353589E-3
			  + $y *(-.2250947176E-3
			  	+ $y * (.6841218299E-5
				  + $y * (0.5824238515E-5
					+ $y * (-.104527497E-5
					  + $y * (.8360937017E-7
						+ $y * (-.3231081277E-8
						  + $y * (.3657763036E-10
							+ $y *.6936233982E-12)))))))))));
	if ($p>.5)
                $x = -$x;
	return $x;
}

function _subuprob ($x) {
	var $p = 0; /* if ($absx > 100) */
	var $absx = Math.abs($x);

	if ($absx < 1.9) {
		$p = Math.pow((1 +
			$absx * (.049867347
			  + $absx * (.0211410061
			  	+ $absx * (.0032776263
				  + $absx * (.0000380036
					+ $absx * (.0000488906
					  + $absx * .000005383)))))), -16)/2;
	} else if ($absx <= 100) {
		for (var $i = 18; $i >= 1; $i--) {
			$p = $i / ($absx + $p);
		}
		$p = Math.exp(-.5 * $absx * $absx) 
			/ Math.sqrt(2 * Math.PI) / ($absx + $p);
	}

	if ($x<0)
        	$p = 1 - $p;
	return $p;
}

   
function _subt ($n, $p) {

	if ($p >= 1 || $p <= 0) {
		throw("Invalid p: $p\n");
	}

	if ($p == 0.5) {
		return 0;
	} else if ($p < 0.5) {
		return - _subt($n, 1 - $p);
	}

	var $u = _subu($p);
	var $u2 = Math.pow($u, 2);

	var $a = ($u2 + 1) / 4;
	var $b = ((5 * $u2 + 16) * $u2 + 3) / 96;
	var $c = (((3 * $u2 + 19) * $u2 + 17) * $u2 - 15) / 384;
	var $d = ((((79 * $u2 + 776) * $u2 + 1482) * $u2 - 1920) * $u2 - 945) 
				/ 92160;
	var $e = (((((27 * $u2 + 339) * $u2 + 930) * $u2 - 1782) * $u2 - 765) * $u2
			+ 17955) / 368640;

	var $x = $u * (1 + ($a + ($b + ($c + ($d + $e / $n) / $n) / $n) / $n) / $n);

	if ($n <= Math.pow(log10($p), 2) + 3) {
		var $round;
		do { 
			var $p1 = _subtprob($n, $x);
			var $n1 = $n + 1;
			var $delta = ($p1 - $p) 
				/ Math.exp(($n1 * Math.log($n1 / ($n + $x * $x)) 
					+ Math.log($n/$n1/2/Math.PI) - 1 
					+ (1/$n1 - 1/$n) / 6) / 2);
			$x += $delta;
			$round = round_to_precision($delta, Math.abs(integer(log10(Math.abs($x))-4)));
		} while (($x) && ($round != 0));
	}
	return $x;
}

function _subtprob ($n, $x) {

	var $a;
        var $b;
	var $w = Math.atan2($x / Math.sqrt($n), 1);
	var $z = Math.pow(Math.cos($w), 2);
	var $y = 1;

	for (var $i = $n-2; $i >= 2; $i -= 2) {
		$y = 1 + ($i-1) / $i * $z * $y;
	} 

	if ($n % 2 == 0) {
		$a = Math.sin($w)/2;
		$b = .5;
	} else {
		$a = ($n == 1) ? 0 : Math.sin($w)*Math.cos($w)/Math.PI;
		$b= .5 + $w/Math.PI;
	}
	return max(0, 1 - $b - $a * $y);
}

function _subf ($n, $m, $p) {
	var $x;

	if ($p >= 1 || $p <= 0) {
		throw("Invalid p: $p\n");
	}

	if ($p == 1) {
		$x = 0;
	} else if ($m == 1) {
		$x = 1 / Math.pow(_subt($n, 0.5 - $p / 2), 2);
	} else if ($n == 1) {
		$x = Math.pow(_subt($m, $p/2), 2);
	} else if ($m == 2) {
		var $u = _subchisqr($m, 1 - $p);
		var $a = $m - 2;
		$x = 1 / ($u / $m * (1 +
			(($u - $a) / 2 +
				(((4 * $u - 11 * $a) * $u + $a * (7 * $m - 10)) / 24 +
					(((2 * $u - 10 * $a) * $u + $a * (17 * $m - 26)) * $u
						- $a * $a * (9 * $m - 6)
					)/48/$n
				)/$n
			)/$n));
	} else if ($n > $m) {
		$x = 1 / _subf2($m, $n, 1 - $p)
	} else {
		$x = _subf2($n, $m, $p)
	}
	return $x;
}

function _subf2 ($n, $m, $p) {
	var $u = _subchisqr($n, $p);
	var $n2 = $n - 2;
	var $x = $u / $n * 
		(1 + 
			(($u - $n2) / 2 + 
				(((4 * $u - 11 * $n2) * $u + $n2 * (7 * $n - 10)) / 24 + 
					(((2 * $u - 10 * $n2) * $u + $n2 * (17 * $n - 26)) * $u 
						- $n2 * $n2 * (9 * $n - 6)) / 48 / $m) / $m) / $m);
	var $delta;
	do {
		var $z = Math.exp(
			(($n+$m) * Math.log(($n+$m) / ($n * $x + $m)) 
				+ ($n - 2) * Math.log($x)
				+ Math.log($n * $m / ($n+$m))
				- Math.log(4 * Math.PI)
				- (1/$n  + 1/$m - 1/($n+$m))/6
			)/2);
		$delta = (_subfprob($n, $m, $x) - $p) / $z;
		$x += $delta;
	} while (Math.abs($delta)>3e-4);
	return $x;
}

function _subchisqr ($n, $p) {
	var $x;

	if (($p > 1) || ($p <= 0)) {
		throw("Invalid p: $p\n");
	} else if ($p == 1){
		$x = 0;
	} else if ($n == 1) {
		$x = Math.pow(_subu($p / 2), 2);
	} else if ($n == 2) {
		$x = -2 * Math.log($p);
	} else {
		var $u = _subu($p);
		var $u2 = $u * $u;

		$x = max(0, $n + Math.sqrt(2 * $n) * $u 
			+ 2/3 * ($u2 - 1)
			+ $u * ($u2 - 7) / 9 / Math.sqrt(2 * $n)
			- 2/405 / $n * ($u2 * (3 *$u2 + 7) - 16));

		if ($n <= 100) {
			var $x0;
                        var $p1;
                        var $z;
			do {
				$x0 = $x;
				if ($x < 0) {
					$p1 = 1;
				} else if ($n>100) {
					$p1 = _subuprob((Math.pow(($x / $n), (1/3)) - (1 - 2/9/$n))
						/ Math.sqrt(2/9/$n));
				} else if ($x>400) {
					$p1 = 0;
				} else {
					var $i0
                                        var $a;
					if (($n % 2) != 0) {
						$p1 = 2 * _subuprob(Math.sqrt($x));
						$a = Math.sqrt(2/Math.PI) * Math.exp(-$x/2) / Math.sqrt($x);
						$i0 = 1;
					} else {
						$p1 = $a = Math.exp(-$x/2);
						$i0 = 2;
					}

					for (var $i = $i0; $i <= $n-2; $i += 2) {
						$a *= $x / $i;
						$p1 += $a;
					}
				}
				$z = Math.exp((($n-1) * Math.log($x/$n) - Math.log(4*Math.PI*$x) 
					+ $n - $x - 1/$n/6) / 2);
				$x += ($p1 - $p) / $z;
				$x = round_to_precision($x, 5);
			} while (($n < 31) && (Math.abs($x0 - $x) > 1e-4));
		}
	}
	return $x;
}

function log10 ($n) {
	return Math.log($n) / Math.log(10);
}
 
function max () {
	var $max = arguments[0];
	for (var $i = 0; i < arguments.length; i++) {
                if ($max < arguments[$i])
                        $max = arguments[$i];
	}	
	return $max;
}

function min () {
	var $min = arguments[0];
	for (var $i = 0; i < arguments.length; i++) {
                if ($min > arguments[$i])
                        $min = arguments[$i];
	}
	return $min;
}

function precision ($x) {
	return Math.abs(integer(log10(Math.abs($x)) - SIGNIFICANT));
}

function precision_string ($x) {
	if ($x) {
		return round_to_precision($x, precision($x));
	} else {
		return "0";
	}
}

function round_to_precision ($x, $p) {
        $x = $x * Math.pow(10, $p);
        $x = Math.round($x);
        return $x / Math.pow(10, $p);
}

function integer ($i) {
        if ($i > 0)
                return Math.floor($i);
        else
                return Math.ceil($i);
}

Hi
Why the tprob(5,0) returns 0?