The value of Rmerge(I) for significantly intense reflections
(see _reflns.threshold_expression) in a given shell.
sumi ( sumj | Ij - <I> | )
Rmerge(I) = --------------------------------
sumi ( sumj <I> )
Ij = the intensity of the jth observation of reflection i
<I> = the mean of the intensities of all observations of
reflection i
sumi is taken over all reflections
sumj is taken over all observations of each reflection.