<div dir="ltr"><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">I believe you have asked this question before.</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><b><br></b></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><b>***ncl-talk is not a statistics forum. ***</b></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><b> ***Please talk to a statistician***</b></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><b></b><br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">====<br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">(1) Possibly:<br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">You could use <a href="http://www.ncl.ucar.edu/Document/Functions/Built-in/esacr.shtml"><b>esacr</b></a> or <a href="http://www.ncl.ucar.edu/Document/Functions/Built-in/esacr_n.shtml"><b>esacr_n</b></a> (esacr_n is available in NCL 6.5.0 and later) on the filtered series to get the lag_1 autocorrelation coefficients.</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">r_sst = esacr(sst,1)</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">r1_sst = r_sst(1) ; scalar; r1_sst(1)</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">r1_sst@long_name = "Lag-1 autocorrelation: sst</div></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">print(r1_sst)<br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"> ; pre(lat,lon,time) ... time is rightmost dimension</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">r_pre = esacr(pre,1) ; r_pre(2,lat,lon,time) <br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">r1_pre = r_pre(1,:,:) ; r1_pre(nlat,mlom)</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">r1_pre@long_name = "Lag-1 autocorrelation: pre"<br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">printVarSummary(r1_pre)</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">printMinMax(r1_pre)</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><pre><span style="font-family:arial,sans-serif"> <br></span></pre><pre><span style="font-family:arial,sans-serif">r1_sst <b>:=</b> conform(r1_pre, r1_sst, -1) ; := overwrite<br>printVarSummary(r1_sst) ; (nlat,mlon)<br><br></span></pre><pre><span style="font-family:arial,sans-serif">r1_sst_pre = r1_sst*r1_pre ; array (grid point-by-grid point) multiply</span></pre><pre><span style="font-family:arial,sans-serif">Ndof = N*(1-r1_sst_pre)/(1+r1_sst_pre), where N is the original sample size of time series<br></span></pre><pre><span style="font-family:arial,sans-serif">printVarSummary(Ndof) ; (nlat,mlon)<br></span></pre><pre><span style="font-family:arial,sans-serif">printMinMax(Ndof,0)<br></span></pre></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">====</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">(2)</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">Perhaps, a better approach:</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit"><br></div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">DOI: 10.1016/S0275-1062(04)90014-8</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">(2004) Determination
of the degree of freedom of digital filtered time series with an
application to the correlation analysis between the length of day and
the Southern oscillation index</div><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">====</div><br><div class="gmail-nova-e-text gmail-nova-e-text--size-xl gmail-nova-e-text--family-sans-serif gmail-nova-e-text--spacing-xs gmail-nova-e-text--color-inherit">Again, it is best to talk with a statistician.<br></div><div class="gmail-content-page-header__meta"><div class="gmail-research-detail-meta"><br></div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Dec 2, 2020 at 5:26 PM dickson mbigi via ncl-talk <<a href="mailto:ncl-talk@mailman.ucar.edu">ncl-talk@mailman.ucar.edu</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">Hi everyone,<div>I have sst(time) and pre(lon,lat,time) low pass filtered data sets. I can calculate spatial correlation, but for the significance test using Student's t- test I need to use effective degrees of freedom because my data have been filtered. I went through NCL website and found ''equivalent sample size'' function which I think is quite different from the effective degrees of freedom. Is there a function or a way to use Effective Degree of freedom to perform significance tests in spatial correlation analysis?</div><div><br></div><div>Ndof =N(1-r1r2)/(1+r1r2), where N is sample size, r1&r2 are the lag-1 autocorrelations of the two time series.</div><div><br></div><div>Thanks in advance.</div><font color="#888888"><div>Dickson.</div></font></div>
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