Quadratic Mean

!NPM versionnpm-imagenpm-url !Build Statustravis-imagetravis-url !Coverage Statuscoveralls-imagecoveralls-url !Dependenciesdependencies-imagedependencies-urlComputes the quadratic mean (root mean square) of an array of values ignoring any values which are not numeric.

## Installation

`$ npm install compute-nanqmean`

For use in the browser, use browserify.

## Usage

To use the module,`var nanqmean = require( 'compute-nanqmean' );`

#### nanqmean( arr )

Computes the quadratic mean (root mean square) ignoring non-numeric values.```
var data = [ 2, 7, NaN, 3, -3, NaN, 9 ];
var mu = nanqmean( data );
// returns ~5.5136
```

## Examples

```
var nanqmean = require( 'compute-nanqmean' );
var data = new Array( 1000 );
for ( var i = 0; i < data.length; i++ ) {
if ( i%5 === 0 ) {
data[ i ] = NaN;
} else {
data[ i ] = Math.random() * 100;
}
}
console.log( nanqmean( data ) );
```

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

`$ node ./examples/index.js`

## Notes

- The algorithm to compute the quadratic mean first calculates the
*L2*norm before dividing by the square root of the`array`

length. This particular implementation attempts to avoid overflow and underflow and is accurate to`<1e-13`

compared to the canonical formula for calculating the root mean square. - The quadratic mean of an
`array`

containing non-numeric values is equal to the quadratic mean of an equivalent`array`

which contains only the numeric values. Hence,

```
var d1 = [ 1, NaN, 2, 3, NaN ],
d2 = [ 1, 2, 3 ];
console.log( nanqmean( d1 ) === nanqmean( d2 ) );
// returns true
```

## References

- Dahlquist, Germund and Bjorck, Ake.
*Numerical Methods in Scientific Computing*. - Blue, James (1978) "A Portable Fortran Program To Find the Euclidean Norm of a Vector".
*ACM Transactions on Mathematical Software*. - Higham, Nicholas J.
*Accuracy and Stability of Numerical Algorithms, Second Edition*.

This module implements a one-pass algorithm proposed by S.J. Hammarling.

## Tests

### Unit

Unit tests use the Mocha test framework with Chai assertions. To run the tests, execute the following command in the top-level application directory:`$ make test`

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

### Test Coverage

This repository uses Istanbul as its code coverage tool. To generate a test coverage report, execute the following command in the top-level application directory:`$ make test-cov`

Istanbul creates a

`./reports/coverage`

directory. To access an HTML version of the report,`$ make view-cov`