[R] Identify period length of time series automatically?

[Rainer M Krug]

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On 14/04/11 11:57, Mike Marchywka wrote:
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>> Date: Thu, 14 Apr 2011 11:29:23 +0200
>> From: r.m.krug at gmail.com
>> To: r-help at r-project.org
>> Subject: [R] Identify period length of time series automatically?
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>> Hi
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>> I have 10.000 simulations for a sensitivity analysis. I have done a few
>> sensitivity analysis for different response variables already,
>> but now, as most of the simulations (if not all) show some cyclic
>> behaviour, see how the independent input parameter influence the
>> frequency of the cyclic changes and "how cyclic" they actually are.
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>> So effectively, I have 39 values, and I want to identify automatically
>> the frequency / period length of the series and a kind of a measure on
>> "how cyclic" the series is.

Hi Mike,

thanks for your answer - it confirms my fears ...

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> Probably google "Digital Signal Processing" or Fourier transform.
> From this, you resolve your time series into sinusoids of various components
> and you can separate peaks in line spectra from background noise. 
> Depending on what you consider to be "cyclic" the analysis details
> will vary. If you look at things like amplitude and frequncy modulation
> of one sine wave with another and various relationships between carrier and
> modulation frequency, you can get some ideas of what to look for in spectra.

That is what I thought as well. As I have no idea about fourier
analysis, could you give me a small example in R, which gives me the
frequencies of the resulting sin waves after a fourier transformation?
I only see large matrices as return values when using e.g. fft().

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> Alternatively, you can try to define exactly what you mean by "cyclic"
> and maybe make a better transform that discriminates that from acyclic
> but offhand I would suggest FFT and various tests on the spectra.

the shape of the fluctuations can be quite different - so no common
pattern there.

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> Just off hand I'm not sure that 39 points would be a lot to go on
> but you can simulate some examples in R quite easily if you know
> what the data looks like in various cases you think may exist.

Well - the data is over a year summed up data from daily data points, so
I could easily go to daily data, which would be 365*39. But that would
make the analysis probably more difficult, as I have seasonal
fluctuations, and fluctuations over several years (1, 2, 3, 4, ...?;
depending on the parameters used for the simulation).

Any ideas on how to do this in R?

I have the feeling, that the quesion id more difficult then I thought...

Rainer


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>> How can I do that automatically without individual checking? I do not
>> want to do an eyeball assessment for 10.000 time series....
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>> Thanks,
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>> Rainer
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>> - --
>> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
>> Biology, UCT), Dipl. Phys. (Germany)
>>
>> Centre of Excellence for Invasion Biology
>> Stellenbosch University
>> South Africa
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- -- 
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
Biology, UCT), Dipl. Phys. (Germany)

Centre of Excellence for Invasion Biology
Stellenbosch University
South Africa

Tel :       +33 - (0)9 53 10 27 44
Cell:       +33 - (0)6 85 62 59 98
Fax :       +33 - (0)9 58 10 27 44

Fax (D):    +49 - (0)3 21 21 25 22 44

email:      Rainer at krugs.de

Skype:      RMkrug
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