Question

does anyone have the inverse chi-square distribution as a PHP function? I basically need the equiv. of excel function CHIINV but useable in PHP.

or can anyone write this function in PHP? big huge thanks to anyone that does.

excel function info taken from microsoft site:
CHIINV(p, df) is the inverse function for CHIDIST(x, df). For any particular x, CHIDIST(x, df) returns the probability that a Chi-Square-distributed random variable with df degrees of freedom is greater than or equal to x.

Answer 1

actually here i did the work:

function AChiSq($p,$n) { 
$v=0.5;
$dv=0.5; 
$x=0;
    while($dv>1e-15) {
        $x=1/$v-1;
        $dv=$dv/2;
        if (ChiSq($x,$n)>$p) {
            $v=$v-$dv;
        }
        else {
            $v=$v+$dv;
        } 
    }
    return $x;
}
function Norm($z) {
    $q=$z*$z;
   if (abs($z)>7)
        return (1-1/$q+3/($q*$q))*exp(-$q/2)/(abs($z)*sqrt($PiD2));
    else
        return ChiSq($q,1);
}
function ChiSq($x,$n) {
    if ($x>1000 || $n>1000) {
            $q=Norm((pow($x/$n,1/3)+2/(9*$n)-1)/sqrt(2/(9*$n)))/2;
            if ($x>$n) 
                return $q;
            else
                return 1-$q; 
        }
    $p=exp(-0.5*$x);
        if(($n%2)==1) { $p=$p*sqrt(2*$x/$Pi);       }
    $k=$n; 
        while($k>=2) {
        $p=$p*$x/$k;
        $k=$k-2;
        }
   $t=$p;
     $a=$n;
     while($t>1e-15*$p) {
        $a=$a+2;
        $t=$t*$x/$a; 
        $p=$p+$t;
        }

    return 1-$p;
}
function calppm($conf,$fails,$total)
{
    $E5 = $conf/100;
  $F5 = $fails;
  $G5 = $total;

    $I5 = AChiSq((1-$E5),(2*($F5+1)))*1000000/(2*$G5);

    return round($I5);
}

//echo calppm(60,0,50000); //18
?>