EDM Quantification of Diffraction
Patterns:
Noise Response dependence on XCF Motif Base Level
The default value for the XCF motif base level
in EDM is 50. We examined the range from 20 to 50. Too small a value implies a less accurate
motif, as even the weak reflections (with lower signal/noise ratio) are used to generate said
motif. We do not want too large a value, because we want to use a statistically significant
number of spots in order to generate a representative motif.
We measured, for each value of XCF motif base level, the number of peaks measured in
both the original and modified patterns. A greater number of peaks will make conclusions
more reliable, but we want to avoid spurious peaks. The result shown to the right is rather
unexpected: this parameter should only determine how many peaks are used to generate the motif,
but all peaks should be measured. So in principle, the total number of reflections should be
roughly constant; this was actually observed for lower values, always around 310 spots. It is
not clear why this goes down in the middle of the range examined here.
All cases shared a total of 260 common reflections, which were the only ones used
to calculate the average relative discrepancies and other figures of merit, as shown
in the plots below.
In the range of values examined, the noise response was very weakly dependent on the
XCF motif base level. The average relative discrepancy, under both definitions used, was quite
flat.
The other three measures fluctuated very little too. The R2 for the "y=x" fit
shows in this range a variation smaller than 10-4. The slope in
the more flexible "y=mx+c" fit also shows minuscule variation, ~2x10-4. The intercept
also changes very little (NOTE ON UNITS?). Nonetheless, it must be mentioned that all these three
measures coincide qualitatively in their pattern, always showing better performance
(albeit only slightly so) around a value of 30-40.