On Mon, 23 Mar 2009, Phil Evans wrote:
I'm happy to change the column titles if it makes it clearer. Actually the
"I/sigma" column in the Scala output is not very useful:
it is <I> / RMSscatter, ie the mean intensity/mean error, for individual
observations, not taking into account multiple measurements. Because it is
ratio of means (rather than a mean of ratios), it can behave oddly depending
on the distribution of intensities, for instance giving an overall value
which is outside the range of values in resolution bins. It is the ratio of
the previous two columns.
On the other hand the column labelled "Mn(I)/sd" is the mean of ratios for
each reflection, ie< <I>/σ(<I>) > and does take into account the
multiplicity of measurements, so is much more relevant as an indicator of
data quality
see
http://www.ccp4wiki.org/~ccp4wiki/wiki/index.php?title=Scaling_experimental_intensities_with_Scala
FWIW, dtscaleaverage in the d*TREK has the same two columns of I/sigI and
labels them "I/sig unavg" and "I/sig avg"
Of course, going from one to the other takes into account the multiplicity
of the observations with the ASSUMPTION that the errors are random and
normally distributed. With all the systematic and erratic errors in
measurements, I'm not sure this assumption is always valid.
Note to students: I think this will be on the quiz I hand out next week!
Jim
Scala also outputs a convenient "Table 1" summary
...
