Isotropy  The property of being
directionally independent, implying no variation of specified physical
properties with direction.

Anisotropy  The property of being
directionally dependent, implying variation of specified physical
properties with direction.

Diffraction limit  The isotropic or
anisotropic limit on average of statistically significant data (also
known as the 'resolution limit'), depending on both the
properties of the crystal and the datacollection strategy.

Threshold  Criterion used to make a decision
on rejection of data.

Spherical shell  Region of reciprocal space
lying between two defined reciprocalspace radii (d*). The
statistics of both isotropic and anisotropic properties are analyzed in
spherical shells.

Resolution cutoff  Cutoff applied to all
diffraction data outside a single closed isotropic or anisotropic
surface in reciprocal space defining the assumed boundary between, on
average, statistically significant and insignificant data. In the
case of an isotropic surface only one parameter is needed to define it,
i.e. the radius of the limiting sphere. In the general case
the surface is anisotropic and one parameter (or even a small number of
parameters) is not sufficient.

Possible data  All reflections inside the
highest resolution cutoff over all directions, regardless of whether or
not an intensity was measured (systematic absences are excluded
everywhere).

Measured data  All possible reflections with
an experimental intensity and standard uncertainty.

Unmeasured data  All possible reflections
without an experimental intensity and standard uncertainty.

Observed data  All measured reflections
inside the resolution cutoff surface and included in the statistical
tables and the reflection output file.

Unobserved data  All measured reflections
outside the resolution cutoff surface and excluded from the statistical
tables and the reflection output file.

Observable data  All unmeasured reflections
inside the resolution cutoff surface (i.e. expected to be
observed).

Unobservable data  All unmeasured
reflections outside the resolution cutoff surface (i.e. expected
to be unobserved).

Fitted ellipsoid  Ellipsoid fitted by a
multidimensional search and weighted least squares algorithm to the
reflection data immediately neighboring the resolution cutoff
surface. Used to reduce the number of parameters required to define
the anisotropic resolution cutoff surface and thereby estimate the
anisotropic resolution limits and predict observable data.

DebyeWaller factor  Attenuation factor of
each measured intensity and its standard uncertainty due to thermal
motion and/or static disorder.

Intensity prior  Expected intensity,
estimated from a spherical intensity distribution obtained from
experimental data or from a model, combined with the symmetry
enhancement factor and the isotropic or anisotropic DebyeWaller factor.

Anisotropic correction factor  The inverse
of the square root of the anisotropic DebyeWaller factor applied as a
multiplicative correction of the amplitudes after Bayesian estimation
from the intensities.

Completeness  Number of data as a fraction
of the number of possible data.
