dvarianceyc

Calculate the variance of a double-precision floating-point strided array using a one-pass algorithm proposed by Youngs and Cramer.

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.

Usage

var dvarianceyc = require( '@stdlib/stats/base/dvarianceyc' );

dvarianceyc( N, correction, x, strideX )

Computes the variance of a double-precision floating-point strided array x using a one-pass algorithm proposed by Youngs and Cramer.

var Float64Array = require( '@stdlib/array/float64' );

var x = new Float64Array( [ 1.0, -2.0, 2.0 ] );

var v = dvarianceyc( 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. 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 Float64Array.
strideX: stride length for x.

The N and stride parameters determine which elements in the strided array are accessed at runtime. For example, to compute the variance of every other element in x,

var Float64Array = require( '@stdlib/array/float64' );

var x = new Float64Array( [ 1.0, 2.0, 2.0, -7.0, -2.0, 3.0, 4.0, 2.0 ] );

var v = dvarianceyc( 4, 1, x, 2 );
// returns 6.25

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

var Float64Array = require( '@stdlib/array/float64' );

var x0 = new Float64Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
var x1 = new Float64Array( x0.buffer, x0.BYTES_PER_ELEMENT*1 ); // start at 2nd element

var v = dvarianceyc( 4, 1, x1, 2 );
// returns 6.25

dvarianceyc.ndarray( N, correction, x, strideX, offsetX )

Computes the variance of a double-precision floating-point strided array using a one-pass algorithm proposed by Youngs and Cramer and alternative indexing semantics.

var Float64Array = require( '@stdlib/array/float64' );

var x = new Float64Array( [ 1.0, -2.0, 2.0 ] );

var v = dvarianceyc.ndarray( x.length, 1, x, 1, 0 );
// returns ~4.33333

The function has the following additional parameters:

offsetX: 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 element in x starting from the second element

var Float64Array = require( '@stdlib/array/float64' );

var x = new Float64Array( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );

var v = dvarianceyc.ndarray( 4, 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), both functions return NaN.

Examples

var discreteUniform = require( '@stdlib/random/array/discrete-uniform' );
var dvarianceyc = require( '@stdlib/stats/base/dvarianceyc' );

var x = discreteUniform( 10, -50, 50, {
    'dtype': 'float64'
});
console.log( x );

var v = dvarianceyc( x.length, 1, x, 1 );
console.log( v );

C APIs

Usage

#include "stdlib/stats/base/dvarianceyc.h"

stdlib_strided_dvarianceyc( N, correction, *X, strideX )

Computes the variance of a double-precision floating-point strided array using a one-pass algorithm proposed by Youngs and Cramer.

const double x[] = { 1.0, -2.0, 2.0 };

double v = stdlib_strided_dvarianceyc( 3, 1.0, x, 1 );
// returns ~4.3333

The function accepts the following arguments:

N: [in] CBLAS_INT number of indexed elements.
correction: [in] double 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. 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: [in] double* input array.
strideX: [in] CBLAS_INT stride length for X.

double stdlib_strided_dvarianceyc( const CBLAS_INT N, const double correction, const double *X, const CBLAS_INT strideX );

stdlib_strided_dvarianceyc_ndarray( N, correction, *X, strideX, offsetX )

Computes the variance of a double-precision floating-point strided array using a one-pass algorithm proposed by Youngs and Cramer and alternative indexing semantics.

const double x[] = { 1.0, -2.0, 2.0 };

double v = stdlib_strided_dvarianceyc_ndarray( 3, 1.0, x, 1, 0 );
// returns ~4.3333

The function accepts the following arguments:

N: [in] CBLAS_INT number of indexed elements.
correction: [in] double 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. 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: [in] double* input array.
strideX: [in] CBLAS_INT stride length for X.
offsetX: [in] CBLAS_INT starting index for X.

double stdlib_strided_dvarianceyc_ndarray( const CBLAS_INT N, const double correction, const double *X, const CBLAS_INT strideX, const CBLAS_INT offsetX );

Examples

#include "stdlib/stats/base/dvarianceyc.h"
#include <stdio.h>

int main( void ) {
    // Create a strided array:
    const double x[] = { 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 };

    // Specify the number of elements:
    const int N = 4;

    // Specify the stride length:
    const int strideX = 2;

    // Compute the variance:
    double v = stdlib_strided_dvarianceyc( N, 1, x, strideX );

    // Print the result:
    printf( "sample variance: %lf\n", v );
}

References

Youngs, Edward A., and Elliot M. Cramer. 1971. "Some Results Relevant to Choice of Sum and Sum-of-Product Algorithms." Technometrics 13 (3): 657–65. doi:10.1080/00401706.1971.10488826.

dvarianceyc

Usage

dvarianceyc( N, correction, x, strideX )

dvarianceyc.ndarray( N, correction, x, strideX, offsetX )

Notes

Examples

C APIs

Usage

stdlib_strided_dvarianceyc( N, correction, *X, strideX )

stdlib_strided_dvarianceyc_ndarray( N, correction, *X, strideX, offsetX )

Examples

References

See Also