Changes between Initial Version and Version 1 of Ticket #217, comment 2


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Timestamp:
Apr 4, 2014 6:09:24 AM (5 years ago)
Author:
butler
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  • Ticket #217, comment 2

    initial v1  
    22SasView assumes the underlying distribution is a number distribution and thereby the reported mean from that distribution is a number average parameter.  It properly weights that distribution by volume when fitting to the scattering data as is required to fit scattering data.   
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    4 However, from the perspective of the Schulz distribution, it is clear that the litterature has conflicting views about what the correct form of the number average Shulz should be.  
     4However, from the perspective of the Schulz distribution, it is clear that the litterature has conflicting views about what the correct form of the number average Schulz should be.  
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    66Fundamentally this means that there are TWO different distributions which are being called the Schulz distribution.  These are unfortunately related through the mass.  
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    8 SasView, like FISH and NIST IGOR macros, take their distribution from the M. Kotlarchyk and S-H. Chen (J. Chem. Phys. 79 (1983) 2461-2469) paper which specifically gives the distribution as being by number.  This is the same number Shulz distrbition given by Aragon and Pecora in their 1976 J Chem Phys paperaper as well as in the 1992 J. Chem Phys paper of Bartlett and Ottewill, among others.  On the other hand, the early papers on this distribution were specifically applied to degrees of polymerization but were clear that they viewed the equivalent formula to be a weight average degree of polymerization.  It seems that at least Welch and Bloomfield in their 1973 j. pol sci paper and Klaus Huber 2012 now use Aragon and Pecora's number average formula but call it a weight average formula and compute a new number avarage formula therefrom. 
     8SasView, like FISH and NIST IGOR macros, take their distribution from the M. Kotlarchyk and S-H. Chen (J. Chem. Phys. 79 (1983) 2461-2469) paper which specifically gives the distribution as being by number.  This is the same number Schulz distrbition given by Aragon and Pecora in their 1976 J Chem Phys paperaper as well as in the 1992 J. Chem Phys paper of Bartlett and Ottewill, among others.  On the other hand, the early papers on this distribution were specifically applied to degrees of polymerization but were clear that they viewed the equivalent formula to be a weight average degree of polymerization.  It seems that at least Welch and Bloomfield in their 1973 J. Pol. Sci. paper and Klaus Huber 2012 now use the same equation as Aragon and Pecora's number average formula but define it as a weight average formula and compute a new number avarage formula therefrom. 
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    1010At the end these are just numerical distributions and can both be used but the question is whether to add another distribution with the same name (which means making sure we distinguish somehow), leave as is based on the fact the the preponderance of usage in scattering seems to be the current SasView version of the distribution, or something else (such as providing different moments of the distribution, or plots of the distribution etc)