Hi, To compare the results what you need to employ is a technique from the statistical field of model selection. The spherical diffusion (isotropic) + all model-free models of all selected residues is one single mathematical model. The prolate and oblate spheroids (prolate and oblate axially symmetric anisotropic diffusion tensors) + all model-free models, and the ellipsoid (fully anisotropic or three different eigenvalues) + all model-free models, are three additional mathematical models. Therefore to compare these four different models you need to select the model which best represents your relaxation data. These models are, however, not nested and therefore cannot be compared using ANOVA F-tests! Firstly the three types of diffusion tensor are not nested (there is a reference from Dominique Marion's group in which they say ANOVA statistics cannot be used but I can't find it at the moment (although it shouldn't be too hard to track down, it's related to Tensor)). Secondly the model-free models selected will be different between the four models. Hence chi-squared and F-tests cannot be used.
A useful reference (I'm not at all biased ;) for this problem is my paper d'Auvergne, E. J. and Gooley, P. R. (2003), (see http://www.nmr-relax.com/refs.html for the full reference). On page 37 at the end of that paper I discuss how AIC model selection is perfect for selecting between these non-nested models. The AIC criterion is still AIC = chi2 + 2k, however chi2 is the minimised chi-squared value for the complete model and k is the sum of the number of diffusion parameters and number of model-free parameters for all spin systems. BIC model selection is likely to work quite well as well. If you have four runs, one for each of the diffusion models, then the relax user function 'model_selection()' is designed to select between these models. I hope this helps and hasn't been too biased. Cheers, Edward P.S. Prior to model selection between the diffusion models, the diffusion models must have fully converged. Multiple iterations of optimisation of the model-free models, AIC model selection, and optimisation of all parameters together (diffusion tensor + model-free parameter of all residues) must be executed. Convergence is when two iterations possess identical chi-squared values, identical model-free models, and identical parameter values. On 3/2/07, Hongyan Li <[EMAIL PROTECTED]> wrote:
Dear relax users, I have managed to use Relax to run my dynamics data by both isotropic and axial-oblate models. My qestion is how to compare the results, by chi-square?? what is the criteria to make judgment that which residues is beter simulated with which model? Thanks for your kind help! Best wishes, Hongyan Dr. Hongyan Li Department of Chemistry The University of Hong Kong Pokfulam Road Hong Kong _______________________________________________ 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
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