[Ifeffit] A basic question about data collection

[Frenkel, Anatoly]

I am probably missing the point, but it is not immediately obvious to me why the following is equivalent in terms of improving the signal to noise: a) constant E-space increment and b) constant k-space increment combined with k-dependent integration time. In a), the data cluster at high E, but each data point in E corresponds to a different final state and thus is unique. Averaging over E-space data in the small interval Delta E, (1/Delta E)*Int [xmu(E) dE] is not equivalent to the time average of xmu(E) collected at a fixed E: (1/T)*Int [xmu(E) dt]. Thus, k^n-weighted integration time, to my mind, is the only proper way of reducing statistical noise. 

Anatoly


-----Original Message-----
From:	ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Carlo Segre
Sent:	Thu 8/25/2005 5:13 PM
To:	XAFS Analysis using Ifeffit
Cc:	
Subject:	Re: [Ifeffit] A basic question about data collection


Matt and Scott:

On Thu, 25 Aug 2005, Matt Newville wrote:

> This is important for QEXAFS (which typically does sample at a
> very fine energy grid). I've been told by people doing QEXAFS
> that a simple box-car average is good enough for binnning QEXAFS
> data.  That's what Ifeffit's rebin() function does. I'd think
> that a more sophisticated rolling average (convolution)  would
> be better (and not screw up energy resolution), but apparantly
> it's not an issue.
>

I have ben playing with the athena smoothing and rebinning funcionalities 
and I think that I prefer the rebinning because smoothing tends to 
attenuate sharp peaked structure.  A rolling average might be good too but 
I haven't tried it too much.  My guess is that for gentle features such as 
in the EXAFS region, rebinning, rolling averages and smoothing will all 
give statistically indistiguishable results.  I amy be wrong.

Carlo

-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
Carlo.Segre at iit.edu    http://www.iit.edu/~segre
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