'use strict';

// MODULES //

var isnan = require( '@stdlib/math/base/assert/is-nan' );


// MAIN //

/**
* Returns the variance of a negative binomial distribution.
*
* @param {PositiveNumber} r - number of failures until experiment is stopped
* @param {Probability} p - success probability
* @returns {NonNegativeNumber} 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
*/
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 );
} // end FUNCTION variance()


// EXPORTS //

module.exports = variance;

'use strict';

// MODULES //

var isnan = require( '@stdlib/math/base/assert/is-nan' );


// MAIN //

/**
* 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
*/
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 );
} // end FUNCTION variance()


// EXPORTS //

module.exports = variance;

Fix return annotation