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Taylor, R., *J. Applied Cryst*., **1986**, 19, 90-91.

For further information on **basic statistical** terms and their calculation refer
to any comprehensive undergraduate textbook, such as:

Snedecor, G.W. & Cochran, W.G., *Statistical Methods*, **1980**, Iowa State
University Press.

For a summary of **principal component analysis** see for example:

Auf der Heyde, T.P.E., *J. Chem. Educ*., **1990**, 67, 461-469.

Chatfield, C. & Collins, A.J., *Introduction to Multivariate Analysis*, **1980**,
London, Chapman and Hall.

**Circular methods** are used to generate descriptive statistics for torsion angles:

Allen, F.H. & Johnson, O., *Acta Cryst*., **1991**, B47, 62-67.

**Conical correction** applies specifically to D-H...A angles in H-bonded systems. For
details of this see:

Stouten, P.F.W., Van Eijck, B.P. & Kroon, J., *J. Mol. Struct*., **1991**, 243,
61-87.