# movstd

Moving standard deviation

## Description

example

M = movstd(A,k) returns an array of local k-point standard deviation values. Each standard deviation is calculated over a sliding window of length k across neighboring elements of A. When k is odd, the window is centered about the element in the current position. When k is even, the window is centered about the current and previous elements. The window size is automatically truncated at the endpoints when there are not enough elements to fill the window. When the window is truncated, the standard deviation is taken over only the elements that fill the window. M is the same size as A.

• If A is a vector, then movstd operates along the length of the vector A.

• If A is a multidimensional array, then movstd operates along the first dimension of A whose size does not equal 1.

example

M = movstd(A,[kb kf]) computes the standard deviation with a window of length kb+kf+1. The calculation includes the element in the current position, kb elements backward, and kf elements forward.

example

M = movstd(___,w) specifies a normalization factor for any of the previous syntaxes. When w = 0 (default), M is normalized by k-1 for window length k. When w = 1, M is normalized by k.

example

M = movstd(___,w,dim) specifies the dimension of A to operate along for any of the previous syntaxes. Always specify the weight w from the previous syntax when specifying dim. For example, movstd(A,k,0,2) operates along the columns of a matrix A, computing the k-element sliding standard deviation for each row. The normalization factor is the default, k-1.

example

M = movstd(___,nanflag) specifies whether to include or omit NaN values in A. For example, movstd(A,k,"omitnan") ignores NaN values when computing each standard deviation. By default, movstd includes NaN values.

example

M = movstd(___,Name,Value) specifies additional parameters for the standard deviation using one or more name-value pair arguments. For example, if x is a time vector, then movstd(A,k,"SamplePoints",x) computes the moving standard deviation relative to the times in x.

## Examples

collapse all

Compute the three-point centered moving standard deviation of a row vector. When there are fewer than three elements in the window at the endpoints, take the standard deviation over the elements that are available.

A = [4 8 6 -1 -2 -3 -1 3 4 5];
M = movstd(A,3)
M = 1×10

2.8284    2.0000    4.7258    4.3589    1.0000    1.0000    3.0551    2.6458    1.0000    0.7071

Compute the three-point trailing moving standard deviation of a row vector. When there are fewer than three elements in the window at the endpoints, take the standard deviation over the elements that are available.

A = [4 8 6 -1 -2 -3 -1 3 4 5];
M = movstd(A,[2 0])
M = 1×10

0    2.8284    2.0000    4.7258    4.3589    1.0000    1.0000    3.0551    2.6458    1.0000

Compute the three-point centered moving standard deviation of a row vector and normalize each standard deviation by the number of elements in the window.

A = [4 8 6 -1 -2 -3 -1 3 4 5];
M = movstd(A,3,1)
M = 1×10

2.0000    1.6330    3.8586    3.5590    0.8165    0.8165    2.4944    2.1602    0.8165    0.5000

Compute the three-point centered moving standard deviation for each row of a matrix. The window starts on the first row, slides horizontally to the end of the row, then moves to the second row, and so on. The dimension argument is two, which slides the window across the columns of A. Always specify the normalization factor when specifying the dimension.

A = [4 8 6; -1 -2 -3; -1 3 4];
M = movstd(A,3,0,2)
M = 3×3

2.8284    2.0000    1.4142
0.7071    1.0000    0.7071
2.8284    2.6458    0.7071

Create a row vector containing NaN values.

A = [4 8 NaN -1 -2 -3 NaN 3 4 5];

Compute the three-point centered moving standard deviation of the vector, excluding NaN values. For windows that contain any NaN value, movstd computes with the non-NaN elements.

M = movstd(A,3,"omitnan")
M = 1×10

2.8284    2.8284    6.3640    0.7071    1.0000    0.7071    4.2426    0.7071    1.0000    0.7071

Compute a 3-hour centered moving standard deviation of the data in A according to the time vector t.

A = [4 8 6 -1 -2 -3];
k = hours(3);
t = datetime(2016,1,1,0,0,0) + hours(0:5)
t = 1x6 datetime
01-Jan-2016 00:00:00   01-Jan-2016 01:00:00   01-Jan-2016 02:00:00   01-Jan-2016 03:00:00   01-Jan-2016 04:00:00   01-Jan-2016 05:00:00

M = movstd(A,k,"SamplePoints",t)
M = 1×6

2.8284    2.0000    4.7258    4.3589    1.0000    0.7071

Compute the three-point centered moving standard deviation of a row vector, but discard any calculation that uses fewer than three points from the output. In other words, return only the standard deviations computed from a full three-element window, discarding endpoint calculations.

A = [4 8 6 -1 -2 -3 -1 3 4 5];
M = 1×8

2.0000    4.7258    4.3589    1.0000    1.0000    3.0551    2.6458    1.0000

## Input Arguments

collapse all

Input array, specified as a vector, matrix, or multidimensional array.

Data Types: single | double | logical

Window length, specified as a numeric or duration scalar. When k is a positive integer scalar, the centered standard deviation includes the element in the current position plus surrounding neighbors.

For example, movstd(A,3) computes an array of local three-point standard deviation values.

Directional window length, specified as a numeric or duration row vector containing two elements. When kb and kf are positive integer scalars, the calculation is over kb+kf+1 elements. The calculation includes the element in the current position, kb elements before the current position, and kf elements after the current position.

For example, movstd(A,[2 1]) computes an array of local four-point standard deviation values.

Weight, specified as one of these values:

• 0 — Normalize by k-1, where k is the window length. If k=1, the weight is k.

• 1 — Normalize by k.

Data Types: single | double

Dimension to operate along, specified as a positive integer scalar. If you do not specify the dimension, then the default is the first array dimension of size greater than 1.

Dimension dim indicates the dimension that movstd operates along, that is, the direction in which the specified window slides.

Consider an m-by-n input matrix, A:

• movstd(A,k,0,1) computes the k-element sliding standard deviation for each column of A and returns an m-by-n matrix.

• movstd(A,k,0,2) computes the k-element sliding standard deviation for each row of A and returns an m-by-n matrix.

Missing value condition, specified as one of these values:

• "includemissing" or "includenan" — Include NaN values in A when computing each standard deviation. If any element in the window is NaN, then the corresponding element in M is NaN. "includemissing" and "includenan" have the same behavior.

• "omitmissing" or "omitnan" — Ignore NaN values in A, and compute each standard deviation over fewer points. If all elements in the window are NaN, then the corresponding element in M is NaN. "omitmissing" and "omitnan" have the same behavior.

### Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: M = movstd(A,k,"Endpoints","fill")

Method to treat windows near endpoints, specified as one of these options:

ValueDescription
"shrink"Shrink the window size near the endpoints of the input to include only existing elements.
"discard"Do not output any standard deviations values when the window does not completely overlap with existing elements.
"fill"Replace nonexisting elements with NaN.
numeric or logical scalarReplace nonexisting elements with the specified numeric or logical value.

Sample points for computing standard deviations, specified as a vector. The sample points represent the locations of the data in A. Sample points do not need to be uniformly sampled. By default, the sample points vector is [1 2 3 ... ].

Moving windows are defined relative to the sample points, which must be sorted and contain unique elements. For example, if t is a vector of times corresponding to the input data, then movstd(rand(1,10),3,"SamplePoints",t) has a window that represents the time interval between t(i)-1.5 and t(i)+1.5.

When the sample points vector has data type datetime or duration, then the moving window length must have type duration.

If the sample points are nonuniformly spaced and Endpoints is specified, then its value must be "shrink".

collapse all

### Standard Deviation

For a finite-length vector A made up of N scalar observations, the standard deviation is defined as

$\begin{array}{l}S={\sqrt{\frac{1}{N-1}\sum _{i=1}^{N}|{A}_{i}-\mu {|}^{2}}}^{},\hfill \\ \hfill \end{array}$

where μ is the mean of A:

$\mu =\frac{1}{N}\sum _{i=1}^{N}{A}_{i}.$

The standard deviation is the square root of the variance. Some definitions of standard deviation use a normalization factor of N instead of N-1, which you can specify by setting w to 1.

## Version History

Introduced in R2016a

expand all