[Ifeffit] statistical parameters
Stefano Ciurli
stefano.ciurli at unibo.it
Thu Dec 15 03:28:48 CST 2005
Hi Matt,
>Ifeffit uses an approximation of 2*delta_k * delta_R / pi.
OK. One nail down.
>Well, it
>could use a better approximation -- see below. But Ifeffit does not
>use 2dKdR/pi + 2. The Stern paper considers "independent points" to
>be discrete, counts the endpoints of this discrete array of
>"independent points", and neglects the rather significant problem of
>the limit dk->0. I find each of these to be problematic. The dk->0
>limit problem makes it especially hard to implement in a meaningful
>way.
OK
>Ifeffit uses n_idp = 2*delta_k * delta_R / pi, but is cautious (and
>probably overly so) in determining delta_k and delta_R. First, it
>puts these on the k- and R- grids (0.05 Ang^-1 and ~0.03067Ang). It
>is also works to avoid rounding incorrectly. Thus, currently your
>ranges:
> kmin=2.0, kmax=12, rmin=1.0, rmax=6.0
>get mapped to
> kmin = 2.05, kmax=12.00 , rmin=1.012427, rmax=5.98525
Now I get it...
>Thus you should have n_idp = 31.4824. You can confirm that with
>'print n_idp' from the command line. It sounds like that's what
>you're getting.
>
>And yes, I do think that having "kmin=2.00" map to "kmin=2.05" for
>this part of the calculation is overly cautious, and I will fix that
>in the next release (and n_idp will equal ~31.64). I could be
>persuaded to just use the user-supplied values, but the round-off
>errors shouldn't matter much.
no, it is not a matter of rounding up or arguing whether the exact
number will change the meaning or the analysis of the fit. I asked
because being new to this I am trying to be rigorous and when I see a
formula in a paper I would naively expect that the same formula is
used to calculate output numbers from a program. Now everything is
clearer. Thanks.
>
>In general, I agree with most of the answers to not take N_idp too
>seriously. I would say to report it to 1 decimal place, just to
>emphasize that it is understood to be an estimate and not a discrete
>number.
will do
>Saying "n_idp=12" implies "I could fit up to 12 parameters
>with confidence". It's easy enough to show this is usually NOT the
>case, but some people sure like a hard and fast rule. And, sadly,
>n_idp does influence the error bars some, so it seems wise to get it
>as close as we can.
ok
>That's sort of rambly, but hope that helps
it does!
Stefano
--
____________________________________________
Stefano Ciurli
Professor of Chemistry
Laboratory of Bioinorganic Chemistry
Department of Agro-Environmental Science and Technology
University of Bologna
Viale Giuseppe Fanin, 40
I-40127 Bologna
Italy
Phone: +39-051-209-6204
Fax: +39-051-209-6203
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ma per seguir virtute e canoscenza"
Dante Alighieri - Inferno - Canto XXVI
"Ihr seid bestimmt, nicht Tieren gleich zu leben,
Nein, Tugend zu erringen und Erkenntnis"
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But virtue to pursue and knowledge high"
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