function variance( r, p ) {
	if (
		isnan( r ) ||
		isnan( p ) ||
		r <= 0.0 ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	return ( ( 1.0-p ) * r ) / ( p*p );
}
Returns the variance of a negative binomial distribution.

@param {PositiveNumber} r - number of failures until experiment is stopped
@param {Probability} p - success probability
@returns {PositiveNumber} variance

@example
var v = variance( 100, 0.2 );
// returns 2000.0

@example
var v = variance( 20, 0.5 );
// returns 40.0

@example
var v = variance( 10.3, 0.8 );
// returns ~3.219

@example
var v = variance( -2, 0.5 );
// returns NaN

@example
var v = variance( 20, 1.1 );
// returns NaN

@example
var v = variance( 20, NaN );
// returns NaN

@example
var v = variance( NaN, 0.5 );
// returns NaN