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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Surrogate data testing</span></span>
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<p><b>Surrogate data testing</b><sup id="cite_ref-Theiler1_1-0" class="reference"><a href="#cite_note-Theiler1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> (or the <i>method of surrogate data</i>) is a statistical <a href="Proof_by_contradiction" title="Proof by contradiction">proof by contradiction</a> technique similar to <a href="Permutation_test" title="Permutation test">permutation tests</a><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> and <a href="Bootstrapping_(statistics)" title="Bootstrapping (statistics)">parametric bootstrapping</a>. It is used to detect <a href="Non-linearity" class="mw-redirect" title="Non-linearity">non-linearity</a> in a <a href="Time_series" title="Time series">time series</a>.<sup id="cite_ref-Galka_3-0" class="reference"><a href="#cite_note-Galka-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> The technique involves specifying a <a href="Null_hypothesis" title="Null hypothesis">null hypothesis</a> <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle H_{0}}">
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</math></span><img src="./43910602a221b7a4c373791f94793e3008622070.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:2.985ex; height:2.509ex;" alt="{\displaystyle H_{0}}" loading="lazy"></span> using <a href="Monte_Carlo_method" title="Monte Carlo method">Monte Carlo</a> methods. A discriminating statistic is then calculated for the original time series and all the surrogate set. If the value of the statistic is significantly different for the original series than for the surrogate set, the null hypothesis is rejected and non-linearity assumed.<sup id="cite_ref-Galka_3-1" class="reference"><a href="#cite_note-Galka-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p><p>The particular surrogate data testing method to be used is directly related to the null hypothesis. Usually this is similar to the following:
<i>The data is a realization of a stationary linear system, whose output has been possibly measured by a monotonically increasing possibly nonlinear (but static) function</i>.<sup id="cite_ref-Theiler1_1-1" class="reference"><a href="#cite_note-Theiler1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Here <i>linear</i> means that each value is linearly dependent on past values or on present and past values of some independent identically distributed (i.i.d.) process, usually also Gaussian. This is equivalent to saying that the process is <a href="ARMA_model" class="mw-redirect" title="ARMA model">ARMA</a> type. In case of fluxes (continuous mappings), linearity of system means that it can be expressed by a linear differential equation. In this hypothesis, the <i>static</i> measurement function is one which depends only on the present value of its argument, not on past ones.
</p>
<div class="mw-heading mw-heading2"><h2 id="Methods">Methods</h2></div>
<p>Many algorithms to generate surrogate data have been proposed. They are usually classified in two groups:<sup id="cite_ref-Theiler2_4-0" class="reference"><a href="#cite_note-Theiler2-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>
</p>
<ul><li><i>Typical realizations</i>: data series are generated as outputs of a well-fitted model to the original data.</li>
<li><i>Constrained realizations</i>: data series are created directly from original data, generally by some suitable transformation of it.</li></ul>
<p>The last surrogate data methods do not depend on a particular model, nor on any parameters, thus they are non-parametric methods. These surrogate data methods are usually based on preserving the linear structure of the original series (for instance, by preserving the <a href="Autocorrelation_function" class="mw-redirect" title="Autocorrelation function">autocorrelation function</a>, or equivalently the <a href="Periodogram" title="Periodogram">periodogram</a>, an estimate of the sample spectrum).<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>
Among constrained realizations methods, the most widely used (and thus could be called the <i>classical methods</i>) are:
</p>
<ol><li>Algorithm 0, or RS (for <i>Random Shuffle</i>):<sup id="cite_ref-Theiler1_1-2" class="reference"><a href="#cite_note-Theiler1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Scheinkman_6-0" class="reference"><a href="#cite_note-Scheinkman-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> New data are created simply by random permutations of the original series. This concept is also used in <a href="Permutation_test" title="Permutation test">permutation tests</a>. The permutations guarantee the same amplitude distribution as the original series, but destroy any temporal correlation that may have been in the original data. This method is associated to the null hypothesis of the data being uncorrelated i.i.d noise (possibly Gaussian and measured by a static nonlinear function).</li>
<li>Algorithm 1, or RP (for <i>Random Phases</i>; also known as FT, for <a href="Fourier_Transform" class="mw-redirect" title="Fourier Transform">Fourier Transform</a>):<sup id="cite_ref-Theiler1_1-3" class="reference"><a href="#cite_note-Theiler1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Osborne_7-0" class="reference"><a href="#cite_note-Osborne-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> In order to preserve the linear correlation (the periodogram) of the series, surrogate data are created by the inverse Fourier Transform of the modules of Fourier Transform of the original data with new (uniformly random) phases. If the surrogates must be real, the Fourier phases must be antisymmetric with respect to the central value of data.</li>
<li>Algorithm 2, or AAFT (for <i>Amplitude Adjusted Fourier Transform</i>):<sup id="cite_ref-Theiler1_1-4" class="reference"><a href="#cite_note-Theiler1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Theiler2_4-1" class="reference"><a href="#cite_note-Theiler2-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> This method has approximately the advantages of the two previous ones: it tries to preserve both the linear structure and the amplitude distribution. This method consists of these steps:
<ul><li>Scaling the data to a Gaussian distribution (<i>Gaussianization</i>).</li>
<li>Performing a RP transformation of the new data.</li>
<li>Finally doing a transformation inverse of the first one (<i>de-Gaussianization</i>).</li></ul>
<dl><dd>The drawback of this method is precisely that the last step changes somewhat the linear structure.</dd></dl></li>
<li>Iterative algorithm 2, or IAAFT (for <i>Iterative Amplitude Adjusted Fourier Transform</i>):<sup id="cite_ref-Schreiber1_8-0" class="reference"><a href="#cite_note-Schreiber1-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> This algorithm is an iterative version of AAFT. The steps are repeated until the autocorrelation function is sufficiently similar to the original, or until there is no change in the amplitudes.</li></ol>
<p>Many other surrogate data methods have been proposed, some based on optimizations to achieve an autocorrelation close to the original one,<sup id="cite_ref-Schreiber2_9-0" class="reference"><a href="#cite_note-Schreiber2-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Schreiber3_10-0" class="reference"><a href="#cite_note-Schreiber3-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Engbert_11-0" class="reference"><a href="#cite_note-Engbert-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> some based on wavelet transform<sup id="cite_ref-Breakspear_12-0" class="reference"><a href="#cite_note-Breakspear-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Keylock1_13-0" class="reference"><a href="#cite_note-Keylock1-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Keylock2_14-0" class="reference"><a href="#cite_note-Keylock2-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> and some capable of dealing with some types of non-stationary data.<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Nakamura_16-0" class="reference"><a href="#cite_note-Nakamura-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Lucio_17-0" class="reference"><a href="#cite_note-Lucio-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup>
</p><p>The above mentioned techniques are called linear surrogate methods, because they are based on a linear process and address a linear null hypothesis.<sup id="cite_ref-Schreiber2_9-1" class="reference"><a href="#cite_note-Schreiber2-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> Broadly speaking, these methods are useful for data showing irregular fluctuations (short-term variabilities) and data with such a behaviour abound in the real world. However, we often observe data with obvious periodicity, for example, annual sunspot numbers, electrocardiogram (ECG) and so on. Time series exhibiting strong periodicities are clearly not consistent with the linear null hypotheses. To tackle this case, some algorithms and null hypotheses have been proposed.<sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Resampling_(statistics)" title="Resampling (statistics)">Resampling (statistics)</a></li>
<li><a href="Permutation_test" title="Permutation test">Permutation test</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-20"><span class="mw-cite-backlink"><b><a href="#cite_ref-20">^</a></b></span> <span class="reference-text"><cite id="CITEREFX._LuoT._NakamuraM._Small2005" class="citation journal cs1">X. Luo; T. Nakamura; M. Small (2005). <a rel="nofollow" class="external text" href="https://doi.org/10.1103/PhysRevE.71.026230">"Surrogate test to distinguish between chaotic and pseudoperiodic time series"</a>. <i>Phys. Rev. E</i>. <b>71</b> (2): 026230. <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/nlin/0404054">nlin/0404054</a></span>. <a href="Bibcode_(identifier)" class="mw-redirect" title="Bibcode (identifier)">Bibcode</a>:<a rel="nofollow" class="external text" href="https://ui.adsabs.harvard.edu/abs/2005PhRvE..71b6230L">2005PhRvE..71b6230L</a>. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1103%2FPhysRevE.71.026230">10.1103/PhysRevE.71.026230</a>. <a href="Hdl_(identifier)" class="mw-redirect" title="Hdl (identifier)">hdl</a>:<a rel="nofollow" class="external text" href="https://hdl.handle.net/10397%2F4828">10397/4828</a>. <a href="PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/15783410">15783410</a>. <a href="S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:35512941">35512941</a>.</cite></span>
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