## The Free-Fourier Duck

For the purposes of cross validation we may chose to omit some of our
data.
If 10% of the reciprocal space data are omitted at random, then the
following images are obtained:

If 10% of the reciprocal space data are omitted in thin spherical
shells, then the following images are obtained:

### Crystallographic Interpretation:

Using *Free-Reflections* is a good validation of coordinate
refinement, but is unwise when calculating maps. Particular care must
be taken when using free-sets in density modification calculations,
as the absence of the free-set can systematically bias the rest of the
data.

More Fourier transforms.
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