Table 1. Real-space measures of map quality tested in this work.

			Expected properties
Method	Symbol	Basis	Perfect map	Random map
Skewness of electron density	skew	High positive density and no negative density in a good map	Positive skewness	Near-zero skewness
Contrast of electron density	c	Solvent and macromolecule have different r.m.s. variation in densities	High contrast	Low contrast
Correlation of local r.m.s. density	r2RMS	Solvent region is contiguous so local r.m.s. is correlated with neighboring local r.m.s.	High correlation	Low correlation
Flatness of electron density	F	Solvent region has nearly flat electron density	High value of flatness	Low value of flatness
Number of regions enclosing high density	Nr	Chains of a macromolecule can be represented by a few connected regions of density	Few (but extended) connected regions	Many short connected regions
Overlap of NCS-related density	ONCS	If NCS is present, NCS-related density is similar	High overlap	Low overlap