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It is important that some basic principles of conformation be taught to students. They are added here for completeness.

*Accuracy* is a measure of the deviation of an observed quantity from its
correct value. Since we never know the truly correct value we don't know how
accurate our data are.

*Precision* is a measure of the experimental uncertainty in a measured
quantity, an indication of its reproducibility. This is what we measure and is
what we indicate in an e.s.d.

*Precision of Measurements* . It is important to stress that a value quoted
as 1.395(4) Å means 1.395 0.0004 Å where means an estimated
standard deviation (e.s.d.) (derived from the inverted least squares matrix used
in refining the structure). If the distribution of errors is normal, then there
is a 99% chance that a given measurement will differ by less than 2.7 times the
e.s.d. from the mean. A bond length of 1.542(7) Å (1.542 Å with an
e.s.d. of 0.007 Å) is probably not significantly different from one measured
at 1.527(7) Å.

**Copyright © 1984, 1997 International Union of
Crystallography**