Variance

Lognormal distribution variance.

The variance for a lognormal random variable is

upper V a r left-parenthesis upper X right-parenthesis equals left-bracket exp left-parenthesis sigma squared right-parenthesis minus 1 right-bracket dot exp left-parenthesis 2 mu plus sigma squared right-parenthesis

where μ is the location parameter and σ > 0 is the scale parameter. According to the definition, the natural logarithm of a random variable from a lognormal distribution follows a normal distribution.

Usage

var variance = require( '@stdlib/math/base/dists/lognormal/variance' );

variance( mu, sigma )

Returns the variance for a lognormal distribution with location mu and scale sigma.

var y = variance( 2.0, 1.0 );
// returns ~255.016

y = variance( 0.0, 1.0 );
// returns ~4.671

y = variance( -1.0, 2.0 );
// returns ~396.04

If provided NaN as any argument, the function returns NaN.

var y = variance( NaN, 1.0 );
// returns NaN

y = variance( 0.0, NaN );
// returns NaN

If provided sigma <= 0, the function returns NaN.

var y = variance( 0.0, 0.0 );
// returns NaN

y = variance( 0.0, -1.0 );
// returns NaN

Examples

var randu = require( '@stdlib/random/base/randu' );
var variance = require( '@stdlib/math/base/dists/lognormal/variance' );

var sigma;
var mu;
var y;
var i;

for ( i = 0; i < 10; i++ ) {
    mu = ( randu()*10.0 ) - 5.0;
    sigma = randu() * 20.0;
    y = variance( mu, sigma );
    console.log( 'µ: %d, σ: %d, Var(X;µ,σ): %d', mu.toFixed( 4 ), sigma.toFixed( 4 ), y.toFixed( 4 ) );
}