[Ifeffit] How to judge if the coordination numbers are different?

[jrkizews at ncsu.edu]

Dear XAS community members,
I am studying the structural change of ferrihydrite as it ages with peat,
which is one kind of organic matter. Totally I have three systems. In
these systems, ferrihydrite/peat ratios are 300, 600, 1200 mmol Fe/kg
peat. Smaller number means more peat in the background with ferrihydrite.
I have collected EXAFS data for these three systems that have aged for
different times. The magnitude portion of the Fourier transformed data
look very much the same within each system, with small variation in the
amplitude. I included two Fe-Fe paths (R=3.075 and 3.376 Å) to fit the
second shell. The coordination numbers of these paths seem to increase for
each system as it ages. Following is some results:
300 mmol/kg              1-day          7-day
               Fe-Fe1    1.6 (0.2)      1.7 (0.2)
               Fe-Fe2    1.7 (0.4)      1.8 (0.4)
600 mmol/kg              1-day          7-day              55-day
               Fe-Fe1    1.8 (0.3)      1.9 (0.3)         1.9 (0.4)
               Fe-Fe2    2.0 (0.3)      1.9 (0.3)         2.1 (0.5)
1200 mmol/kg             1-day          7-day              55-day
               Fe-Fe1    1.8 (0.3)      1.9 (0.3)         2.1 (0.4)
               Fe-Fe2    2.0 (0.3)      2.0 (0.2)         2.0 (0.5)
I was told that fitting results of coordination number have big error.
When I fit these data, I put all data sets together. These data share a
lot of common variables such as coordination number of first shell Fe-O
path. The bottom line is that the total number of variables are much
smaller than the independent points, which increases the accuracy of
fitting.
The fitting results coordination numbers are different. However, I do not
know if the difference is real difference, or they are the same within
error, or statistically they are just the same. Could anybody share your
knowledge with me?
Regards.
Fiona Kizewski
North Carolina State Univerity