Geometric Mean
=====
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> Computes the [geometric mean](http://en.wikipedia.org/wiki/Geometric_mean) of an array.


## Installation

``` bash
$ npm install compute-gmean
```

## Usage

``` javascript
var gmean = require( 'compute-gmean' );
```

#### gmean( arr[, accessor] )

Computes the [geometric mean](http://en.wikipedia.org/wiki/Geometric_mean) of an `array`. For numeric `arrays`,

``` javascript
var data = [ 1, 5, 2, 3, 7 ];

var mu = gmean( data );
// returns ~2.914
```

For non-numeric `arrays`, provide an accessor `function` for accessing numeric values

``` javascript
var arr = [
	{'x':1},
	{'x':5},
	{'x':2},
	{'x':3},
	{'x':7}
]:

function getValue( d ) {
	return d.x;
}

var value = prod( arr, getvalue );
// returns ~2.194
```



## Examples

``` javascript
var gmean = require( 'compute-gmean' );

var data = new Array( 1000 );
for ( var i = 0; i < data.length; i++ ) {
	data[ i ] = Math.random() * 100;
}

console.log( gmean( data ) );
```

To run the example code from the top-level application directory,

``` bash
$ node ./examples/index.js
```



## Notes

1. Only calculate the [geometric mean](http://en.wikipedia.org/wiki/Geometric_mean) of an `array` of __positive__ numbers. The textbook formula for calculating the geometric mean involves taking the product of all `array` elements. If one element is `0`, then the product is `0`, even if all other values are `>>> 0`, yielding a nonsensical geometric mean (and measure of the central tendency). Nonsensical results also arise when an `array` contains negative values leading to a product without positive roots and a geometric mean which does not map to the measure's geometric interpretation. For more information, see *Handbook of Parametric and Nonparametric Statistical Procedures: Third Edition* by David J. Sheskin.
2. If an `array` contains values less than or equal to `0`, the function returns `NaN`.
3. If provided an empty `array`, the function returns `null`.
4. For arrays exceeding memory constraints, you are encouraged to use streams; see [flow-gmean](https://github.com/flow-io/flow-gmean).


## Tests

### Unit

Unit tests use the [Mocha](http://mochajs.org) test framework with [Chai](http://chaijs.com) assertions. To run the tests, execute the following command in the top-level application directory:

``` bash
$ make test
```

All new feature development should have corresponding unit tests to validate correct functionality.


### Test Coverage

This repository uses [Istanbul](https://github.com/gotwarlost/istanbul) as its code coverage tool. To generate a test coverage report, execute the following command in the top-level application directory:

``` bash
$ make test-cov
```

Istanbul creates a `./reports/coverage` directory. To access an HTML version of the report,

``` bash
$ make view-cov
```


---
## License

[MIT license](http://opensource.org/licenses/MIT). 

## Copyright

Copyright &copy; 2014-2015. Rebekah Smith.


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