## README

# snanvariancetk

Calculate the variance of a single-precision floating-point strided array ignoring

`NaN`

values and using a one-pass textbook algorithm.

The population variance of a finite size population of size `N`

is given by

where the population mean is given by

Often in the analysis of data, the true population variance is not known *a priori* and must be estimated from a sample drawn from the population distribution. If one attempts to use the formula for the population variance, the result is biased and yields a **biased sample variance**. To compute an **unbiased sample variance** for a sample of size `n`

,

where the sample mean is given by

The use of the term `n-1`

is commonly referred to as Bessel's correction. Note, however, that applying Bessel's correction can increase the mean squared error between the sample variance and population variance. Depending on the characteristics of the population distribution, other correction factors (e.g., `n-1.5`

, `n+1`

, etc) can yield better estimators.

## Installation

```
npm install @stdlib/stats-base-snanvariancetk
```

## Usage

```
var snanvariancetk = require( '@stdlib/stats-base-snanvariancetk' );
```

#### snanvariancetk( N, correction, x, stride )

Computes the variance of a single-precision floating-point strided array `x`

ignoring `NaN`

values and using a one-pass textbook algorithm.

```
var Float32Array = require( '@stdlib/array-float32' );
var x = new Float32Array( [ 1.0, -2.0, NaN, 2.0 ] );
var v = snanvariancetk( x.length, 1, x, 1 );
// returns ~4.3333
```

The function has the following parameters:

**N**: number of indexed elements.**correction**: degrees of freedom adjustment. Setting this parameter to a value other than`0`

has the effect of adjusting the divisor during the calculation of the variance according to`n-c`

where`c`

corresponds to the provided degrees of freedom adjustment and`n`

corresponds to the number of non-`NaN`

indexed elements. When computing the variance of a population, setting this parameter to`0`

is the standard choice (i.e., the provided array contains data constituting an entire population). When computing the unbiased sample variance, setting this parameter to`1`

is the standard choice (i.e., the provided array contains data sampled from a larger population; this is commonly referred to as Bessel's correction).**x**: input`Float32Array`

.**stride**: index increment for`x`

.

The `N`

and `stride`

parameters determine which elements in `x`

are accessed at runtime. For example, to compute the variance of every other element in `x`

,

```
var Float32Array = require( '@stdlib/array-float32' );
var floor = require( '@stdlib/math-base-special-floor' );
var x = new Float32Array( [ 1.0, 2.0, 2.0, -7.0, -2.0, 3.0, 4.0, 2.0, NaN ] );
var N = floor( x.length / 2 );
var v = snanvariancetk( N, 1, x, 2 );
// returns 6.25
```

Note that indexing is relative to the first index. To introduce an offset, use `typed array`

views.

```
var Float32Array = require( '@stdlib/array-float32' );
var floor = require( '@stdlib/math-base-special-floor' );
var x0 = new Float32Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0, NaN ] );
var x1 = new Float32Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element
var N = floor( x0.length / 2 );
var v = snanvariancetk( N, 1, x1, 2 );
// returns 6.25
```

#### snanvariancetk.ndarray( N, correction, x, stride, offset )

Computes the variance of a single-precision floating-point strided array ignoring `NaN`

values and using a one-pass textbook algorithm and alternative indexing semantics.

```
var Float32Array = require( '@stdlib/array-float32' );
var x = new Float32Array( [ 1.0, -2.0, NaN, 2.0 ] );
var v = snanvariancetk.ndarray( x.length, 1, x, 1, 0 );
// returns ~4.33333
```

The function has the following additional parameters:

**offset**: starting index for`x`

.

While `typed array`

views mandate a view offset based on the underlying `buffer`

, the `offset`

parameter supports indexing semantics based on a starting index. For example, to calculate the variance for every other value in `x`

starting from the second value

```
var Float32Array = require( '@stdlib/array-float32' );
var floor = require( '@stdlib/math-base-special-floor' );
var x = new Float32Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
var N = floor( x.length / 2 );
var v = snanvariancetk.ndarray( N, 1, x, 2, 1 );
// returns 6.25
```

## Notes

- If
`N <= 0`

, both functions return`NaN`

. - If
`n - c`

is less than or equal to`0`

(where`c`

corresponds to the provided degrees of freedom adjustment and`n`

corresponds to the number of non-`NaN`

indexed elements), both functions return`NaN`

. - Some caution should be exercised when using the one-pass textbook algorithm. Literature overwhelmingly discourages the algorithm's use for two reasons: 1) the lack of safeguards against underflow and overflow and 2) the risk of catastrophic cancellation when subtracting the two sums if the sums are large and the variance small. These concerns have merit; however, the one-pass textbook algorithm should not be dismissed outright. For data distributions with a moderately large standard deviation to mean ratio (i.e.,
**coefficient of variation**), the one-pass textbook algorithm may be acceptable, especially when performance is paramount and some precision loss is acceptable (including a risk of returning a negative variance due to floating-point rounding errors!). In short, no single "best" algorithm for computing the variance exists. The "best" algorithm depends on the underlying data distribution, your performance requirements, and your minimum precision requirements. When evaluating which algorithm to use, consider the relative pros and cons, and choose the algorithm which best serves your needs.

## Examples

```
var randu = require( '@stdlib/random-base-randu' );
var round = require( '@stdlib/math-base-special-round' );
var Float32Array = require( '@stdlib/array-float32' );
var snanvariancetk = require( '@stdlib/stats-base-snanvariancetk' );
var x;
var i;
x = new Float32Array( 10 );
for ( i = 0; i < x.length; i++ ) {
x[ i ] = round( (randu()*100.0) - 50.0 );
}
console.log( x );
var v = snanvariancetk( x.length, 1, x, 1 );
console.log( v );
```

## References

- Ling, Robert F. 1974. "Comparison of Several Algorithms for Computing Sample Means and Variances."
*Journal of the American Statistical Association*69 (348). American Statistical Association, Taylor & Francis, Ltd.: 859–66. doi:10.2307/2286154.

## Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

#### Community

## License

See LICENSE.

## Copyright

Copyright © 2016-2021. The Stdlib Authors.