rathinaswamy bavanandan govindan rbg26471 at YAHOO.COM
Mon Jun 27 15:51:28 CEST 2005

Dear Litvak,
I am not using Fieldtrip, but I can try to address
your query. Subtracting one spectrum from other is not
a correct way to compare two spectra. On the
otherhand, you can consider the ratio of the two

Further, in the spectral analysis, no one is
interested in the power in all the frequency bands but
in a specific frequency band! So, in that case you can
calculate the signal to noise ratio (SNR) for both
processes and compare the SNR of the two processes!

You don't have to search for a complicated formula to
compute SNR. Just sum up the powers in the desired
frequency range and divide by the sum of powers in all
the frequencies up to Nyquist frequency. In this
definition care should be taken to avoid the effects
due to filter. If you are using bandpass filter, say
up to 80 Hz, then restrict your second summation only
up to the frequency band of 80 Hz.

Best regards
--- Litvak Vladimir <litvak at TECHUNIX.TECHNION.AC.IL>

> Dear Robert and Eric,
> Maybe I should explain what I really want to do and
> then you'll be able to
> suggest another solution for me.
> I want to compare power between two conditions A and
> B (dependent
> samples). I already tried comparing it the usual way
> but I want to also
> try comparing relative increase (A-B)/B.  This way I
> have only one dataset
> that needs to be compared to zero. If I apply the
> same transformation to B
> I'll get of course just an array of zeros and it
> doesn't seem right to me
> to compare it with the normalized A.
> Any ideas? Maybe I should just normalize both by the
> average 2A/(A+B) and
> 2B/(A+B) and the compare?
> Thanks,
> Vladimir

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