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R-symm is a monitor of data quality during X-ray diffraction data collection. It represents the extent to which observations of each unique reflection deviate from being equal to each other. The percentage deviation is indeed termed Rsymm. As a precise monitor of data quality, Rsymm is flawed (they say) but however it is widely used. Also, often people use the term R-merge when they mean Rsymm. 

It's preferable to use R-symm when referring to scaling within the same crystal:

It is preferable to use the R-merge when merging datasets from multiple crystals, that is equivalent to the R-Factor for the comparison of N datasets after merging:

where |Fhkl| is the final value of the structure factor amplitude.

On I(hkl):

R-merge is also found as:

and often where indeed people want to refer to R-symm.

Rpim=Precision-indicating merging R-value

A 1/(N-1) term is introduced in the linear merging R-value. This indicator will decrease with redundancy N, and it will reflect the increasing accuracy (precision) of intensities as more and more observations are merged (the linear R-value suggests the opposite).

For Tables in manuscripts use:

Rsym = Σhkl Σi |Ii(hkl) - <I(hkl)>| / Σhkl Σi Ii(hkl)
Rmerge = Σ (I -<I>) / Σ<I>