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Topic: Non linear association
Replies: 2   Last Post: Jan 20, 2013 1:24 PM

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Daniel Robbins

Posts: 27
Registered: 6/12/12
Re: Non linear association
Posted: Jan 20, 2013 1:24 PM
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Hi Greg,

Thanks for the feedback - very helpful!

If I understand your suggestions for my third query correctly, you are using an approach similar to standard or absolute deviations to quantify the amount of variance in a signal? This could be useful for quantifying the consistency of signals; however I am looking to see if the patterns of change in two signals follow a similar style and timescale.

I have been playing with some code to calculate the first derivative of the signals which can then be analysed statistically. Would you say this is a valid approach?

Thanks again
Dan

"Greg Heath" <heath@alumni.brown.edu> wrote in message <kdgp44$o93$1@newscl01ah.mathworks.com>...
> "Daniel Robbins" <d.w.e.robbins@gmail.com> wrote in message <kdcenb$s4n$1@newscl01ah.mathworks.com>...
> > Hi,
> >
> > I am trying to establish the best way to check the similarity of the pattern of change in two signals/vectors. For a simple example I generated four datasets. One positive parabolic curve, one equal negative curve, one smaller positive curve with a time lag and one vector with a fixed value e.g. [6 6 6 6 6 6 ...etc]. All vectors were the same lengths.
> >
> > My thinking is that I should use some sort of cross correlation e.g.
> >
> > [Rxy, Lag] = xcorr(data(:,4),data(:,3));
> > plot(Lag,Rxy)
> >
> > By changing the two signals compared I note that the peak of the graph changes location. Therefore I could use the x axis values to calculate the lag in the signal. However I don't understand the values on the Y-axis. I also note that if I input the fixed value vector the graph becomes platykurtic, I'm not sure how to interpret this.
> >
> > Please can someone help me with the following queries:
> >
> > 1.) How do I interpret the Y-values of the graph?

>
> If you standardize those columns of data, then the two autocorrelation functions
> will have unity peaks at zero lag.
>
> Then you can compare the peak of the crosscorrelation function at zero lag with unity
> and how many lags it takes the function to decay to some fraction of the max.
>

> > 2.) Is there some way of quantifying the level of agreement in the form of liner correlations e.g. from -1 to 1? Or via p-values?
>
> Most likely. However, that probably isn't the best approach.
>

> > 3.) Is this the best approach to analyse the pattern of change in two non linear signals or is there a better approach?
>
> I would choose sqrt(mean((y1-y2).^2)), mean(abs(y1-y2)) or max(abs(y1-y2))
>

> > Many thanks
>
> You're welcome.
>
> Greg




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