(let ((g (* 2 (or (gethash word good) 0)))
     (b (or (gethash word bad) 0))) 
       (unless (< (+ g b) 5) 
 (max .01 
    (min .99 (float (/ (min 1 (/ b nbad)) 
    (+ (min 1 (/ g ngood)) 
       (min 1 (/ b nbad))))))))

and then . . .

(let ((prod (apply #'* probs))) 
   (/ prod 
      (+ prod 
      (apply #'* 
        (mapcar #'(lambda (x) (- 1 x)) probs))))) 


##</code><code>##

use List::Util qw(min max reduce);
sub score {
    my($word) = @_;
    # uses global %good, %bad, $ngood, $nbad
    my $g = 2 * ($good{$word} || 0);
    my $b = $bad{$word} || 0;
    unless($g + $b < 5) {
       return max(0.01, min (0.99, min (1, $b / $nbad)/
         (min(1, $g / $ngood) + min (1, $b / $nbad))));
    }
    # otherwise: return undef
}

sub prob {
    my @probs = @_;
    my $prod = reduce { $a * $b } @probs;
    return $prod / ($prod + reduce { $a * $b } map { 1 - $_ } @probs);
}