<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title></title><style type="text/css">.felamimail-body-blockquote {margin: 5px 10px 0 3px;padding-left: 10px;border-left: 2px solid #000088;} </style></head><body>Hello<br><br>Indeed i was<span style="font-size:12.8px;"> regressing a EOF principal component timeseries on a detrended time x lat x lon array (temperature). </span><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><span style="font-size:12.8px;">However, i managed to make that work by setting the right metadata.</span> <br></div></div></blockquote><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><span style="font-size:12.8px;"><br></span></div></div></blockquote><p>Now is for the calculation of significance of trends.</p><p>I have some questions regarding the code below, posted next to the line in the code:<br></p><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><span style="font-size:12.8px;"><br></span></div><div><span style="font-size:12.8px;">Here is some code that I use to calculate the significance of trends (using the betainc function as you were doing):</span></div><div><span style="font-size:12.8px;"><br></span></div><div><div><span style="font-size:12.8px;">tarr = obs_ann(:,:,5::12) =====>what is the data format? [lat, lon, time?]<br></span></div><div><span style="font-size:12.8px;">tttt = dtrend_msg(ispan(0,dimsizes(tarr&time)-1,1),tarr,True,True) ====>Are you using the </span><span style="font-size:12.8px;"><span style="font-size:12.8px;">dtrend_msg function because it returns attributes like @slope which is what i need for graphical purposes? </span>why not just use dtrend function?<br></span></div></div></div></blockquote><p><br></p><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><div><span style="font-size:12.8px;">finobs = tarr(0,:,:) ====>have you rearranged the inital data here?<br></span></div><div><span style="font-size:12.8px;">finobs_rtt = finobs</span></div><div><span style="font-size:12.8px;">finobs = (/ onedtond(tttt@slope, (/dimsizes(obs_seas&lat),dimsizes(obs_seas&lon)/) ) /) ===>i understand we get the slope from the detrend function and not from </span><span style="font-size:12.8px;"><span style="font-size:12.8px;">regcoef?</span></span></div><div><span style="font-size:12.8px;"> </span></div><div><span style="font-size:12.8px;">tval = tarr(:,:,0) ====>is this still arranged as lat, lon, time?<br></span></div><div><span style="font-size:12.8px;">df = new((/dimsizes(tarr&lat),dimsizes(tarr&lon)/),"integer") =====>defining new variable===degrees of freedom<br></span></div></div></div></blockquote><p> </p><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><div><span style="font-size:12.8px;"><br></span></div><div><span style="font-size:12.8px;">rc = regcoef(ispan(0,dimsizes(tarr&time)-1,1),tarr,tval,df) ======> i dont understand what is happening here? from NCL online, if you want to calculate trends https://www.ncl.ucar.edu/Applications/Scripts/regress_4.ncl shows to use </span><br><pre>;************************************************
; Calculate the regression coefficients (slopes)
;************************************************
rc = regCoef(year,xann(lat|:,lon|:,year|:))
rc@long_name = "Trend"
rc@units = xann@units+"/year"
copy_VarCoords(xann(0,:,:), rc) ; copy lat,lon coords
printVarSummary(rc)<br></pre></div></div></div></blockquote><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><div><pre><br></pre><span style="font-size:12.8px;"></span></div><div><span style="font-size:12.8px;">df = equiv_sample_size(tarr,0.05,0) ; Optional! over rule df returned from regcoef. U<span style="color:rgb(34,34,34);font-family:arial, sans-serif;font-size:12.8px;font-style:normal;font-weight:400;letter-spacing:normal;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;background-color:rgb(255,255,255);float:none;">p to you whether you want to use equiv_sample_size or not</span></span></div><div><span style="font-size:12.8px;">df = df-2<span style="white-space:pre;"> </span>;regcoef/equiv_sample_size return N, need N-2</span></div><div><span style="font-size:12.8px;">beta_b = tval</span></div><div><span style="font-size:12.8px;">beta_b = 0.5</span></div><div><span style="font-size:12.8px;">finobs_rtt = (/ (1.-betainc(df/(df+tval^2), df/2.0, beta_b))*100. /) ====> is this ttest? with a 2 tailed probability?<br></span></div><div><span style="font-size:12.8px;"><br></span></div></div></div></blockquote><p>Would appreciate is someone would help guide me on this.</p><p>The idea is to just to do a simple trend analysis on temperature from 2020-2100 (80years) and see if its significant or not?</p><p><br></p><blockquote class="felamimail-body-blockquote"><div dir="ltr"><div><div><span style="font-size:12.8px;"><br></span></div><br><div><br></div></div><div><span style="font-size:12.8px;"><br></span></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Mar 5, 2018 at 2:36 AM, Sri Nandini <span dir="ltr"><<a href="mailto:snandini@marum.de" target="_blank">snandini@marum.de</a>></span> wrote:<br><blockquote class="felamimail-body-blockquote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div>Hello<br><br>I am trying to <span class="m_-1226483225063639934st"><em>calculate statistical significance</em> of a <em>linear</em> trend (Regression analysis).<br>The regression analysis simple enough.<br><br>I have troubles understanding how the probabilities are computed based on the trend i calculate?<br>What i actually wanted to do is calculate the 2 tailed ttest probabilities but it doesn't seem possible with NCL, unless someone else knows of a way?<br>Or does </span><span class="m_-1226483225063639934st"><em class="m_-1226483225063639934quotelev3">using the NCL function "betainc" to get the p-value
for a student test is the same thing?<br><br></em>
<em class="m_-1226483225063639934quotelev3">>>></em>My code is below:<br><br><br>f = addfile ("<a href="http://air.mon.mean.nc" target="_blank">air.mon.mean.nc</a>", "r")<br> latS = 29.3<br> latN = 67.26 <br> lonL = 19.<br> lonR = 60.<br><br> TIME1 = f->time<br> YYYY1 = cd_calendar(TIME1,-1)/100 ; entire file<br> iYYYY1 = ind(YYYY1.ge.yrStrt .and. YYYY1.le.yrLast)<br> t2m = f->air(iYYYY1,:,:)<br> printVarSummary(t2m)<br><br> T2M = month_to_season (t2m, season)<br> printVarSummary(T2M)<br> nyrs = dimsizes(T2M&time)<br><br> t2mw = T2M({lat|latS:latN},{lon|lonL:lonR},time|:)<br> printVarSummary(t2mw)<br><br> t2mw = dtrend(t2mw,False)<br> printVarSummary(t2mw)<br><br> eofT = eofunc_Wrap(t2mw, neof, optEOF)<br> printVarSummary(eofT)<br><br>;==========================================================================<br>eof_regres= eofT ;create an array with meta data <br>do ne=0,neof-1<br>eof_regres(ne,:,:)= regCoef(eof_ts(ne,:), t2mw(lat|:,lon|:,time|:));<br> end do<br> printVarSummary(eof_regres)<br><br>;==========================================================================<br>;Calculate significance at 95% confidence<br>;The regCoef function returns the following attributes: yintercept (y-intercept), tval (t-statistic), rstd (standard error of the estimated regression coefficient) and nptxy (number of elements used). <br>;==========================================================================<br><br> df = onedtond(eof_regres@nptxy,dimsizes(eof_regres))-2 ; degrees of freedom<br> tval = onedtond(eof_regres@tval, dimsizes(eof_regres)) ; t-statistic<br><br> b = tval ; b must be same size as tval (and df)<br> b = 0.5<br> prob = betainc(df/(df+tval^2),df/2.0,b)<br> printVarSummary(prob) ; prob(nlat,nlon)<br><br> ;copy_VarCoords(eof_regres,prob)<br> prob@long_name = "probability"<br> ;prob@units = "fraction: [0,1]"<br><br> prob_95=where (prob.ge.0.95,1,0) ; **** note 0.95 ****<br> copy_VarCoords(prob, prob_95)<br> printVarSummary(prob_95) <br><br> eof_regres@long_name = "regression coefficient"<br> prob@long_name = "probability"<br> <br>;==================================The error message i get is:<br><br>warning:VarVarWrite: rhs has no dimension name or coordinate variable, deleting name of lhs dimension number (1) and destroying coordinate var, use "(/../)" if this is not desired outcome<br>warning:VarVarWrite: rhs has no dimension name or coordinate variable, deleting name of lhs dimension number (2) and destroying coordinate var, use "(/../)" if this is not desired outcome<br>warning:["Execute.c":8640]:Execute: Error occurred at or near line 135 in file NAOregreT2M_obs.ncl<br><br><br>Variable: eof_regres<br>Type: float<br>Total Size: 49856 bytes<br> 12464 values<br>Number of Dimensions: 3<br>Dimensions and sizes: [evn | 2] x [76] x [82]<br>Coordinates: <br> evn: [1..2]<br>Number Of Attributes: 11<br> tval : <ARRAY of 6232 elements><br> yintercept : <ARRAY of 6232 elements><br> rstd : <ARRAY of 6232 elements><br> nptxy : <ARRAY of 6232 elements><br> long_name : EOF: DJF: Monthly mean of surface temperature<br> _FillValue : -9.96921e+36<br> method : transpose<br> matrix : covariance<br> pcvar : ( 59.16475, 20.32462 )<br> eval : ( 18982.65, 6521.03 )<br> eval_transpose : ( 169.488, 58.22348 )<br><br><br></span></div>
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