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[ferret_users] how to calculate confidence level in fft

dear ferret users,
1. I want to find the peak signals in the JJAS time,  how many record in input file to get the significant peak of 90 days in the amplitude spectrum? I use the code below, what it means  FFT_nyquist?
2. After evaluating FFT and calculate confidence Level need to check the statistical significance of the peaks, whether any option in ferret
3. If one year spectrum is below confidence how to make  the ensemble of the JJAS amplitude spectrum for 10 years.

thanks in advance for the help
with regards
Senior Research Fellow,
Theoretical Studies Division
IITM, Pune-8

USE "2008_mjjaso.nc"
LET FFT_etfn = sst[y=-3.6:-5.2@ave,x=60:90@ave]
LET FFT_etfft = FFTA(FFT_etfn[l=1:183])
!  Get the frequency increment used in the FFT.
LET FFT_nf = `FFT_etfft,return=lend`
LET FFT_nyquist = 0.5
LET FFT_freq1 = FFT_nyquist/ FFT_nf

!  Define a frequency axis.
DEFINE AXIS/T=`FFT_freq1`:`FFT_nyquist`:`FFT_freq1` faxis
DEFINE GRID/T=faxis gfftfreq
LET a = T[g=gfftfreq]
!  Define the period from the frequency axis.
LET per = 1./a
PLOT/VS/nolab/thick=3/LINE=7/HLIMITS=0:90:10/SET_UP/vlimits=0:0.5:0.05 per[l=1:`FFT_nf`], FFT_etfft

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