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# incrstdev

> Compute a [corrected sample standard deviation][sample-stdev] incrementally.

<section class="intro">

The [corrected sample standard deviation][sample-stdev] is defined as

<!-- <equation class="equation" label="eq:corrected_sample_standard_deviation" align="center" raw="s = \sqrt{\frac{1}{n-1} \sum_{i=0}^{n-1} ( x_i - \bar{x} )^2}" alt="Equation for the corrected sample standard deviation."> -->

<div class="equation" align="center" data-raw-text="s = \sqrt{\frac{1}{n-1} \sum_{i=0}^{n-1} ( x_i - \bar{x} )^2}" data-equation="eq:corrected_sample_standard_deviation">
    <img src="https://cdn.rawgit.com/stdlib-js/stdlib/27e2a43c70db648bb5bbc3fd0cdee050c25adc0b/lib/node_modules/@stdlib/stats/incr/stdev/docs/img/equation_corrected_sample_standard_deviation.svg" alt="Equation for the corrected sample standard deviation.">
    <br>
</div>

<!-- </equation> -->

</section>

<!-- /.intro -->

<section class="usage">

## Usage

```javascript
var incrstdev = require( '@stdlib/stats/incr/stdev' );
```

#### incrstdev( \[mean] )

Returns an accumulator `function` which incrementally computes a [corrected sample standard deviation][sample-stdev].

```javascript
var accumulator = incrstdev();
```

If the mean is already known, provide a `mean` argument.

```javascript
var accumulator = incrstdev( 3.0 );
```

#### accumulator( \[x] )

If provided an input value `x`, the accumulator function returns an updated [corrected sample standard deviation][sample-stdev]. If not provided an input value `x`, the accumulator function returns the current [corrected sample standard deviation][sample-stdev].

```javascript
var accumulator = incrstdev();

var s = accumulator( 2.0 );
// returns 0.0

s = accumulator( 1.0 ); // => sqrt(((2-1.5)^2+(1-1.5)^2) / (2-1))
// returns ~0.7071

s = accumulator( 3.0 ); // => sqrt(((2-2)^2+(1-2)^2+(3-2)^2) / (3-1))
// returns 1.0

s = accumulator();
// returns 1.0
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   Input values are **not** type checked. If provided `NaN` or a value which, when used in computations, results in `NaN`, the accumulated value is `NaN` for **all** future invocations. If non-numeric inputs are possible, you are advised to type check and handle accordingly **before** passing the value to the accumulator function.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var randu = require( '@stdlib/random/base/randu' );
var incrstdev = require( '@stdlib/stats/incr/stdev' );

var accumulator;
var v;
var i;

// Initialize an accumulator:
accumulator = incrstdev();

// For each simulated datum, update the sample standard deviation...
for ( i = 0; i < 100; i++ ) {
    v = randu() * 100.0;
    accumulator( v );
}
console.log( accumulator() );
```

</section>

<!-- /.examples -->

<section class="links">

[sample-stdev]: https://en.wikipedia.org/wiki/Standard_deviation

</section>

<!-- /.links -->
