# Standard Deviation

Erlang distribution standard deviation.

The standard deviation for an Erlang random variable is

where k is the shape parameter and λ is the rate parameter.

## Usage

var stdev = require( '@stdlib/math/base/dists/erlang/stdev' );


#### stdev( k, lambda )

Returns the standard deviation of an Erlang distribution with parameters k (shape parameter) and lambda (rate parameter).

var v = stdev( 1, 1.0 );
// returns 1.0

v = stdev( 4, 12.0 );
// returns ~0.167

v = stdev( 8, 2.0 );
// returns ~1.414


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

var v = stdev( NaN, 2.0 );
// returns NaN

v = stdev( 2.0, NaN );
// returns NaN


If not provided a positive integer for k, the function returns NaN.

var v = stdev( 1.8, 1.0 );
// returns NaN

v = stdev( -1.0, 1.0 );
// returns NaN


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

var v = stdev( 2, 0.0 );
// returns NaN

v = stdev( 2, -1.0 );
// returns NaN


## Examples

var randu = require( '@stdlib/random/base/randu' );
var round = require( '@stdlib/math/base/special/round' );
var EPS = require( '@stdlib/constants/math/float64-eps' );
var stdev = require( '@stdlib/math/base/dists/erlang/stdev' );

var lambda;
var k;
var v;
var i;

for ( i = 0; i < 10; i++ ) {
k = round( randu()*10.0 );
lambda = ( randu()*10.0 ) + EPS;
v = stdev( k, lambda );
console.log( 'k: %d, λ: %d, SD(X;k,λ): %d', k.toFixed( 4 ), lambda.toFixed( 4 ), v.toFixed( 4 ) );
}