[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

"Fatti non foste a viver come bruti,
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"
"Ye were not form'd to live the life of brutes,
But virtue to pursue and knowledge high"



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