# TimeseriesSurrogates.jl

`TimeseriesSurrogates`

is a Julia package for generating surrogate timeseries. It is part of JuliaDynamics, a GitHub organization dedicated to creating high quality scientific software.

If you are new to this method of surrogate timeseries, feel free to read the Crash-course into timeseries surrogate tests page.

Please note that timeseries surrogates should not be confused with surrogate models, such as those provided by Surrogates.jl.

## Installation

TimeseriesSurrogates.jl is a registered Julia package. To install the latest version, run the following code:

`import Pkg; Pkg.add("TimeseriesSurrogates")`

## API

TimeseriesSurrogates.jl API is composed by four names: `surrogate`

, `surrogenerator`

, `SurrogateTest`

, and `pvalue`

. They dispatch on the method to generate surrogates, which is a subtype of `Surrogate`

.

It is recommended to standardize the signal before using these functions, i.e. subtract mean and divide by standard deviation. The function `standardize`

does this.

### Generating surrogates

`TimeseriesSurrogates.surrogate`

— Function`surrogate(x, method::Surrogate [, rng]) → s`

Create a single surrogate timeseries `s`

from `x`

based on the given `method`

. If you want to generate multiple surrogates from `x`

, you should use `surrogenerator`

for better performance.

`TimeseriesSurrogates.surrogenerator`

— Function`surrogenerator(x, method::Surrogate [, rng]) → sgen::SurrogateGenerator`

Initialize a generator that creates surrogates of `x`

on demand, based on the given `method`

. This is more efficient than `surrogate`

, because for most methods some things can be initialized and reused for every surrogate. Optionally you can provide an `rng::AbstractRNG`

object that will control the random number generation and hence establish reproducibility of the generated surrogates. By default `Random.default_rng()`

is used.

The generated surrogates overwrite, in-place, a common vector container. Use `copy`

if you need to actually store multiple surrogates.

To generate a surrogate, call `sgen`

as a function with no arguments, e.g.:

```
sgen = surrogenerator(x, method)
s = sgen()
```

You can use the generator syntax of Julia to map over surrogates generated by `sg`

. For example, let `q`

be a function returning a discriminatory statistic. To test some null hypothesis with TimeseriesSurrogates.jl you'd do

```
using TimeseriesSurrogates
q, x # inputs
method = RandomFourier() # some example method
sgen = surrogenerator(x, method)
siter = (sgen() for _ in 1:1000)
qx = q(x)
qs = map(q, siter)
# compare `qx` with quantiles
using Statistics: quantile
q01, q99 = quantile(qs, [0.01, 0.99])
q01 ≤ qx ≤ q99 # if false, hypothesis can be rejected!
```

### Hypothesis testing

`TimeseriesSurrogates.SurrogateTest`

— Type`SurrogateTest(f::Function, x, method::Surrogate; kwargs...)`

Initialize a surrogate test for input data `x`

, which can be used in `pvalue`

. The tests requires as input a function `f`

that given a timeseries (like `x`

) it outputs a real number, and a method of how to generate surrogates. `f`

is the function that computes the discriminatory statistic.

Once called with `pvalue`

, the test stores the real value `rval`

and surrogate values `vals`

of the discriminatory statistic in the fields `rval, vals`

respectively.

`SurrogateTest`

automates the process described in the documentation page Performing surrogate hypothesis tests.

`SurrogateTest`

subtypes `HypothesisTest`

and is part of the StatsAPI.jl interface.

**Keywords**

`rng = Random.default_rng()`

: a random number generator.`n::Int = 10_000`

: how many surrogates to generate and compute`f`

on.

`StatsAPI.pvalue`

— Method`pvalue(test::SurrogateTest; tail = :left)`

Return the p-value corresponding to the given `SurrogateTest`

, optionally specifying what kind of tail test to do (one of `:left, :right, :both`

).

The default value of `tail`

assumes that the surrogate data are expected to have higher discriminatory statistic values. This is the case for statistics that quantify entropy. For statistics that quantify autocorrelation, use `tail = :right`

instead.

## Surrogate methods

`TimeseriesSurrogates.Surrogate`

— TypeSupertype of all surrogate methods.

`TimeseriesSurrogates.AAFT`

`TimeseriesSurrogates.BlockShuffle`

`TimeseriesSurrogates.CircShift`

`TimeseriesSurrogates.CycleShuffle`

`TimeseriesSurrogates.IAAFT`

`TimeseriesSurrogates.PartialRandomization`

`TimeseriesSurrogates.PartialRandomizationAAFT`

`TimeseriesSurrogates.PseudoPeriodic`

`TimeseriesSurrogates.PseudoPeriodicTwin`

`TimeseriesSurrogates.RandomCascade`

`TimeseriesSurrogates.RandomFourier`

`TimeseriesSurrogates.RandomShuffle`

`TimeseriesSurrogates.Surrogate`

`TimeseriesSurrogates.SurrogateTest`

`TimeseriesSurrogates.TAAFT`

`TimeseriesSurrogates.TFTD`

`TimeseriesSurrogates.TFTDAAFT`

`TimeseriesSurrogates.TFTDIAAFT`

`TimeseriesSurrogates.TFTDRandomFourier`

`TimeseriesSurrogates.TFTS`

`TimeseriesSurrogates.WLS`

### Shuffle-based

`TimeseriesSurrogates.RandomShuffle`

— Type`RandomShuffle() <: Surrogate`

A random constrained surrogate, generated by shifting values around.

Random shuffle surrogates preserve the mean, variance and amplitude distribution of the original signal. Properties not preserved are *any temporal information*, such as the power spectrum and hence linear correlations.

The null hypothesis this method can test for is whether the data are uncorrelated noise, possibly measured via a nonlinear function. Specifically, random shuffle surrogate can test the null hypothesis that the original signal is produced by independent and identically distributed random variables[^Theiler1991, ^Lancaster2018].

*Beware: random shuffle surrogates do not cover the case of correlated noise*^{[Lancaster2018]}.

`TimeseriesSurrogates.BlockShuffle`

— Type`BlockShuffle(n::Int; shift = false)`

A block shuffle surrogate constructed by dividing the time series into `n`

blocks of roughly equal width at random indices (end blocks are wrapped around to the start of the time series).

If `shift`

is `true`

, then the input signal is circularly shifted by a random number of steps prior to picking blocks.

Block shuffle surrogates roughly preserve short-range temporal properties in the time series (e.g. correlations at lags less than the block length), but break any long-term dynamical information (e.g. correlations beyond the block length).

Hence, these surrogates can be used to test any null hypothesis aimed at comparing short-range dynamical properties versus long-range dynamical properties of the signal.

`TimeseriesSurrogates.CycleShuffle`

— Type`CycleShuffle(n::Int = 7, σ = 0.5)`

Cycle shuffled surrogates^{[Theiler1994]} that identify successive local peaks in the data and shuffle the cycles in-between the peaks. Similar to `BlockShuffle`

, but here the "blocks" are defined as follows:

- The timeseries is smoothened via convolution with a Gaussian (
`DSP.gaussian(n, σ)`

). - Local maxima of the smoothened signal define the peaks, and thus the blocks in between them.
- The first and last index of timeseries can never be peaks and thus signals that should have peaks very close to start or end of the timeseries may not perform well. In addition, points before the first or after the last peak are never shuffled.
- The defined blocks are randomly shuffled as in
`BlockShuffle`

.

CSS are used to test the null hypothesis that the signal is generated by a periodic oscillator with no dynamical correlation between cycles, i.e. the evolution of cycles is not deterministic.

See also `PseudoPeriodic`

.

`TimeseriesSurrogates.CircShift`

— Type`CircShift(n)`

Surrogates that are circularly shifted versions of the original timeseries.

`n`

can be an integer (the surrogate is the original time series shifted by `n`

indices), or any vector of integers, which which means that each surrogate is shifted by an integer selected randomly among the entries in `n`

.

### Fourier-based

`TimeseriesSurrogates.RandomFourier`

— Type`RandomFourier(phases = true)`

A surrogate that randomizes the Fourier components of the signal in some manner. If `phases==true`

, the phases are randomized, otherwise the amplitudes are randomized. `FT`

is an alias for `RandomFourier`

.

Random Fourier phase surrogates^{[Theiler1991]} preserve the autocorrelation function, or power spectrum, of the original signal. Random Fourier amplitude surrogates preserve the mean and autocorrelation function but do not preserve the variance of the original. Random amplitude surrogates are not common in the literature, but are provided for convenience.

Random phase surrogates can be used to test the null hypothesis that the original signal was produced by a linear Gaussian process ^{[Theiler1991]}.

`TimeseriesSurrogates.TFTDRandomFourier`

— Type`TFTD(phases::Bool = true, fϵ = 0.05)`

The `TFTDRandomFourier`

(or just `TFTD`

for short) surrogate was proposed by Lucio et al. (2012)^{[Lucio2012]} as a combination of truncated Fourier surrogates^{[Nakamura2006]} (`TFTS`

) and detrend-retrend surrogates.

The `TFTD`

part of the name comes from the fact that it uses a combination of truncated Fourier transforms (TFT) and de-trending and re-trending (D) the time series before and after surrogate generation. Hence, it can be used to generate surrogates also from (strongly) nonstationary time series.

**Implementation details**

Here, a best-fit linear trend is removed/added from the signal prior to and after generating the random Fourier signal. In principle, any trend can be removed, but so far, we only provide the linear option.

`TimeseriesSurrogates.PartialRandomization`

— Type`PartialRandomization(α = 0.5)`

`PartialRandomization`

surrogates^{[Ortega1998]} are similar to `RandomFourier`

phase surrogates, but during the phase randomization step, instead of drawing phases from `[0, 2π]`

, phases are drawn from `[0, 2π]*α`

, where `α ∈ [0, 1]`

. The authors refers to `α`

as the "degree" of phase randomization, where `α = 0`

means `0 %`

randomization and `α = 1`

means `100 %`

randomization.

`TimeseriesSurrogates.PartialRandomizationAAFT`

— Type`PartialRandomizationAAFT(α = 0.5)`

`PartialRandomizationAAFF`

surrogates are similar to `PartialRandomization`

surrogates^{[Ortega1998]}, but adds a rescaling step, so that the surrogate has the same values as the original time series (analogous to the rescaling done for `AAFT`

surrogates). Partial randomization surrogates have, to the package authors' knowledge, not been published in scientific literature.

`TimeseriesSurrogates.AAFT`

— Type`AAFT()`

An amplitude-adjusted-fourier-transform (AAFT) surrogate^{[Theiler1991]}.

AAFT surrogates have the same linear correlation, or periodogram, and also preserves the amplitude distribution of the original data.

AAFT surrogates can be used to test the null hypothesis that the data come from a monotonic nonlinear transformation of a linear Gaussian process (also called integrated white noise)^{[Theiler1991]}.

`TimeseriesSurrogates.TAAFT`

— Type`TAAFT(fϵ)`

An truncated version of the amplitude-adjusted-fourier-transform surrogate^{[Theiler1991]}^{[Nakamura2006]}.

The truncation parameter and phase randomization procedure is identical to `TFTS`

, but here an additional step of rescaling back to the original data is performed. This preserves the amplitude distribution of the original data.

`TimeseriesSurrogates.IAAFT`

— Type`IAAFT(M = 100, tol = 1e-6, W = 75)`

An iteratively adjusted amplitude-adjusted-fourier-transform surrogate^{[SchreiberSchmitz1996]}.

IAAFT surrogates have the same linear correlation, or periodogram, and also preserves the amplitude distribution of the original data, but are improved relative to AAFT through iterative adjustment (which runs for a maximum of `M`

steps). During the iterative adjustment, the periodograms of the original signal and the surrogate are coarse-grained and the powers are averaged over `W`

equal-width frequency bins. The iteration procedure ends when the relative deviation between the periodograms is less than `tol`

(or when `M`

is reached).

IAAFT, just as AAFT, can be used to test the null hypothesis that the data come from a monotonic nonlinear transformation of a linear Gaussian process.

### Non-stationary

`TimeseriesSurrogates.TFTS`

— Type`TFTS(fϵ::Real)`

A truncated Fourier transform surrogate^{[Nakamura2006]} (TFTS).

TFTS surrogates are generated by leaving some frequencies untouched when performing the phase shuffling step (as opposed to randomizing all frequencies, like for `RandomFourier`

surrogates).

These surrogates were designed to deal with data with irregular fluctuations superimposed over long term trends (by preserving low frequencies)^{[Nakamura2006]}. Hence, TFTS surrogates can be used to test the null hypothesis that the signal is a stationary linear system generated the irregular fluctuations part of the signal^{[Nakamura2006]}.

**Controlling the truncation of the spectrum**

The truncation parameter `fϵ ∈ [-1, 0) ∪ (0, 1]`

controls which parts of the spectrum are preserved.

- If
`fϵ > 0`

, then`fϵ`

indicates the ratio of high frequency domain to the entire frequency domain. For example,`fϵ = 0.5`

preserves 50% of the frequency domain (randomizing the higher frequencies, leaving low frequencies intact). - If
`fϵ < 0`

, then`fϵ`

indicates ratio of low frequency domain to the entire frequency domain. For example,`fϵ = -0.2`

preserves 20% of the frequency domain (leaving higher frequencies intact, randomizing the lower frequencies). - If
`fϵ ± 1`

, then all frequencies are randomized. The method is then equivalent to`RandomFourier`

.

The appropriate value of `fϵ`

strongly depends on the data and time series length, and must be manually determined^{[Nakamura2006]}, for example by comparing periodograms for the time series and the surrogates.

`TimeseriesSurrogates.TFTD`

— Type`TFTD(phases::Bool = true, fϵ = 0.05)`

The `TFTDRandomFourier`

(or just `TFTD`

for short) surrogate was proposed by Lucio et al. (2012)^{[Lucio2012]} as a combination of truncated Fourier surrogates^{[Nakamura2006]} (`TFTS`

) and detrend-retrend surrogates.

The `TFTD`

part of the name comes from the fact that it uses a combination of truncated Fourier transforms (TFT) and de-trending and re-trending (D) the time series before and after surrogate generation. Hence, it can be used to generate surrogates also from (strongly) nonstationary time series.

**Implementation details**

Here, a best-fit linear trend is removed/added from the signal prior to and after generating the random Fourier signal. In principle, any trend can be removed, but so far, we only provide the linear option.

`TimeseriesSurrogates.TFTDAAFT`

— Type`TFTDAAFT(fϵ = 0.05)`

`TFTDAAFT`

^{[Lucio2012]} are similar to `TFTD`

surrogates, but also re-scales back to the original values of the time series. `fϵ ∈ (0, 1]`

is the fraction of the powerspectrum corresponding to the lowermost frequencies to be preserved.

`TimeseriesSurrogates.TFTDIAAFT`

— Type`TFTDIAAFT(fϵ = 0.05; M::Int = 100, tol::Real = 1e-6, W::Int = 75)`

`TFTDIAAFT`

^{[Lucio2012]} are similar to `TFTDAAFT`

, but adds an iterative procedure to better match the periodograms of the surrogate and the original time series, analogously to how `IAAFT`

improves upon `AAFT`

.

`fϵ ∈ (0, 1]`

is the fraction of the powerspectrum corresponding to the lowermost frequencies to be preserved. `M`

is the maximum number of iterations. `tol`

is the desired maximum relative tolerance between power spectra. `W`

is the number of bins into which the periodograms are binned when comparing across iterations.

### Pseudo-periodic

`TimeseriesSurrogates.PseudoPeriodic`

— Type`PseudoPeriodic(d, τ, ρ, shift = true)`

Create surrogates suitable for pseudo-periodic signals. They retain the periodic structure of the signal, while inter-cycle dynamics that are either deterministic or correlated noise are destroyed (for appropriate `ρ`

choice). Therefore these surrogates are suitable to test the null hypothesis that the signal is a periodic orbit with uncorrelated noise^{[Small2001]}.

Arguments `d, τ, ρ`

are as in the paper, the embedding dimension, delay time and noise radius. The method works by performing a delay coordinates embedding from DelayEmbeddings.jl (see that docs for choosing appropriate `d, τ`

). For `ρ`

, we have implemented the method proposed in the paper in the function `noiseradius`

.

The argument `shift`

is not discussed in the paper. If `shift=false`

we adjust the algorithm so that there is little phase shift between the periodic component of the original and surrogate data.

See also `CycleShuffle`

.

`TimeseriesSurrogates.PseudoPeriodicTwin`

— Type```
PseudoPeriodicTwin(d::Int, τ::Int, δ = 0.2, ρ = 0.1, metric = Euclidean())
PseudoPeriodicTwin(δ = 0.2, ρ = 0.1, metric = Euclidean())
```

A pseudoperiodic twin surrogate^{[Miralles2015]}, which is a fusion of the twin surrogate^{[Thiel2006]} and the pseudo-periodic surrogate^{[Small2001]}.

**Input parameters**

A delay reconstruction of the input timeseries is constructed using embedding dimension `d`

and embedding delay `τ`

. The threshold `δ ∈ (0, 1]`

determines which points are "close" (neighbors) or not, and is expressed as a fraction of the attractor diameter, as determined by the input data. The authors of the original twin surrogate paper recommend `0.05 ≤ δ ≤ 0.2`

^{[Thiel2006]}.

If you have pre-embedded your timeseries, and timeseries is already a `::StateSpaceSet`

, use the three-argument constructor (so that no delay reconstruction is performed). If you want a surrogate for a scalar-valued timeseries, use the five-argument constructor to also provide the embedding delay `τ`

and embedding dimension `d`

.

**Null hypothesis**

Pseudo-periodic twin surrogates generate signals similar to the original data if the original signal is (quasi-)periodic. If the original signal is not (quasi-)periodic, then these surrogates will have different recurrence plots than the original signal, but preserve the overall shape of the attractor. Thus, `PseudoPeriodicTwin`

surrogates can be used to test null hypothesis that the observed timeseries (or orbit) is consistent with a quasi-periodic orbit^{[Miralles2015]}.

**Returns**

A `d`

-dimensional surrogate orbit (a `StateSpaceSet`

) is returned. Sample the first column of this dataset if a scalar-valued surrogate is desired.

### Wavelet-based

`TimeseriesSurrogates.WLS`

— Type```
WLS(shufflemethod::Surrogate = IAAFT();
f::Union{Nothing, Function} = Statistics.cor,
rescale::Bool = true,
wt::Wavelets.WT.OrthoWaveletClass = Wavelets.WT.Daubechies{16}())
```

A wavelet surrogate generated by the following procedure:

- Compute the wavelet transform of the signal. This results in a set of detail coefficients over a set of dyadic scales. As in Keylock (2006), we here use the maximal overlap discrete wavelet transform, or MODWT, so that the number of coefficients at each scale are the same.
- Shuffle the detail coefficients at each dyadic scale using the provided
`shufflemethod`

. See "Shuffling methods" below for alternatives. - Apply the inverse wavelet transform to the shuffled detail coefficients to obtain a surrogate time series.

**Shuffling methods**

You may choose to use any surrogate from this package to perform the randomization of the detail coefficients at each dyadic scale.

The following methods have been discussed in the literature (more may exist):

- Random permutations of wavelet coefficients within each scale (Breakspear et al., 2003). To get this behaviour, use
`WLS(x, RandomShuffle(), rescale = false, f = nothing)`

. - Cyclic rotation of wavelet coefficients within each scale (Breakspear et al., 2003). To get this behaviour, use
`WLS(x, Circshift(1:length(x)), rescale = false, f = nothing)`

. - Block resampling of wavelet coefficients within each scale (Breakspear et al., 2003). To get this behaviour, use
`WLS(x, BlockShuffle(nblocks, randomize = true), rescale = false, f = nothing)`

. - IAAFT resampling of wavelet coefficients within each scale (Keylock, 2006). To get this behaviour, use
`WLS(x, IAAFT(), rescale = true, f = Statistics.cor)`

. This method preserves the local mean and variance structure of the signal, but randomises nonlinear properties of the signal (i.e. Hurst exponents)^{[Keylock2006]}. These surrogates can therefore be used to test for changes in nonlinear properties of the original signal. In contrast to IAAFT surrogates, the IAAFT-wavelet surrogates also preserves nonstationarity. Using other`shufflemethod`

s does not necessarily preserve nonstationarity. To deal with nonstationary signals, Keylock (2006) recommends using a wavelet with a high number of vanishing moments. Thus, our default is to use a Daubechies wavelet with 16 vanishing moments.*Note: The iterative procedure after the rank ordering step (step [v] in*^{[Keylock2006]}) is not performed in this implementation.

The default method and parameters replicate the behaviour of Keylock (2006)'s IAAFT wavelet surrogates.

**Error minimization**

For the `IAAFT`

approach introduced in Keylock (2006), detail coefficients at each level are circularly rotated to minimize an error function. The methods introduced in Breakspear et al. (2003) do not apply this error minimization.

In our implementation, you can turn this option on/off using the `f`

parameter of the `WLS`

constructor. If `f = nothing`

turns off error minization. If `f`

is set to a two-argument function that computes some statistic, for example `f = Statistics.cor`

, then detail coefficients at each scale are circularly rotated until that function is maximized (and hence the "error" minimized). If you want to *minimize* some error function, then instead provide an appropriate transform of your function. For example, if using the root mean squared deviation, define `rmsd_inv(x, y) = 1 - StatsBase.rmsd(x, y)`

and set `f = rmsd_inv`

.

**Rescaling**

If `rescale == true`

, then surrogate values are mapped onto the values of the original time series, as in the `AAFT`

algorithm. If `rescale == false`

, surrogate values are not constrained to the original time series values. If `AAFT`

or `IAAFT`

shuffling is used, `rescale`

should be set to `true`

. For other methods, it does not necessarily need to be.

`TimeseriesSurrogates.RandomCascade`

— Type`RandomCascade(paddingmode::String = "zeros")`

A random cascade multifractal wavelet surrogate (Paluš, 2008)^{[Paluš2008]}.

If the input signal length is not a power of 2, the signal must be padded before the surrogate is constructed. `paddingmode`

determines how the signal is padded. Currently supported padding modes: `"zeros"`

. The final surrogate (constructed from the padded signal) is subset to match the length of the original signal.

Random cascade surrogate preserve multifractal properties of the input time series, that is, interactions among dyadic scales and nonlinear dependencies^{[Paluš2008]}.

### Other

```
AutoRegressive
ShuffleDimensions
IrregularLombScargle
```

### Utilities

`TimeseriesSurrogates.noiseradius`

— Function`noiseradius(x::AbstractVector, d::Int, τ, ρs, n = 1) → ρ`

Use the proposed* algorithm of^{[Small2001]} to estimate optimal `ρ`

value for `PseudoPeriodic`

surrogates, where `ρs`

is a vector of possible `ρ`

values. *The paper is ambiguous about exactly what to calculate. Here we count how many times we have pairs of length-2 that are identical in `x`

and its surrogate, but **are not** also part of pairs of length-3.

This function directly returns the arg-maximum of the evaluated distribution of these counts versus `ρ`

, use `TimeseriesSurrogates._noiseradius`

with same arguments to get the actual distribution. `n`

means to repeat τhe evaluation `n`

times, which increases accuracy.

## Visualization

TimeseriesSurrogates.jl has defined a simple function `surroplot(x, s)`

. This comes into scope when `using Makie`

(you also need a plotting backend).

To load the function, do:

```
using TimeseriesSurrogates
using CairoMakie, Makie
using TimeseriesSurrogates, CairoMakie, Makie
ts = AR1() # create a realization of a random AR(1) process
s = surrogate(ts, AAFT())
fig = surroplot(ts, s)
save("surroplot.png", fig); # hide
```

## Citing

Please use the following BiBTeX entry, or DOI, to cite TimeseriesSurrogates.jl:

DOI: https://doi.org/10.21105/joss.04414

BiBTeX:

```
@article{TimeseriesSurrogates.jl,
doi = {10.21105/joss.04414},
url = {https://doi.org/10.21105/joss.04414},
year = {2022},
publisher = {The Open Journal},
volume = {7},
number = {77},
pages = {4414},
author = {Kristian Agasøster Haaga and George Datseris},
title = {TimeseriesSurrogates.jl: a Julia package for generating surrogate data},
journal = {Journal of Open Source Software}
}
```

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