Hi, Concerning the AIC value, you're absolutely right. A negligible (-> 0) Rex should yield the same chi2 as a nested model, but a AIC value different by 2...
Regards, Séb Edward d'Auvergne wrote: > On Tue, Mar 10, 2009 at 6:23 PM, Sébastien Morin > <[email protected]> wrote: > >> Hi Ed, >> >> PY is next to me and we discussed this issue together. >> >> This seems very strange since values of the AICc criterion (extracted >> during model selection) are not equal for models with or without these >> 10e-19 Rex... If this is in s-1, then the criterion should be almost >> equal as a Rex of 10e-19 s-1 would be negligible. >> > > For this value, see below. If Rex really is zero between 2 nested > models where the Rex parameter is the only difference, the AIC values > should be different by 2! The reason is because as: > > AIC = chi^2 + 2k, > > if Rex = 0, then the other parameters must be the same (unless of a > model or optimisation failure) and the chi-squared values equal. > Hence as the parameter number difference k_diff = 1, then AIC_diff = > 2. > > > >> Moreover, if using the latex_mf_table.py script, the Rex values are >> reported and associated to the 800 MHz field strength with values in s-1 >> between 1 and 6... These values correspond to those we would obtain by >> multiplying the Rex values in the xml results file by (2 pi frq)**2... >> >> Hence, I would think values extracted in the xml file, that is values >> one gets with the command "value.display('Rex')", are in the field >> independent format obtained from Rex/omega**2. >> >> Are we wrong ? >> > > I just checked the code in the math_fns package, and the chemical > exchange is handled internally as Rex/omega**2. Looking at the XML > dumping code, this just dumps the contents of the spin container > directly to file, hence the value is Rex/omega**2. Therefore the > comment in the XML results file is incorrect. Would you be able to > file a bug report for this? Then the fixes can be tracked. It would > be better to have two reports, one for the latex_mf_table.py script > and one for the XML results file. > > Cheers, > > Edward > > -- Sébastien Morin PhD Student S. Gagné NMR Laboratory Université Laval & PROTEO Québec, Canada _______________________________________________ relax (http://nmr-relax.com) This is the relax-users mailing list [email protected] To unsubscribe from this list, get a password reminder, or change your subscription options, visit the list information page at https://mail.gna.org/listinfo/relax-users
