Statistical descriptors in crystallography

Historical background - 1984-1989

A result of several discussions at the XIII International Congress of Crystallography in Hamburg 1984, particularly those arising at a microsymposium devoted to crystallographic statistics, was a renewed recognition of the wide nonuniformity in use and nomenclature of many statistical methods applied to crystallography. The Commission on Crystallographic Nomenclature addressed this problem soon after the Congress had ended and agreed that an attempt should be made to improve the situation. Accordingly, a Subcommittee on Statistical Descriptors in Crystallography was appointed in early 1985 with its terms of reference 'to examine the validity of current statistical approaches used in estimating the variances in crystallographic quantities and to make recommendations for an improved methodology that rests securely on sound modern statistical theory and that can be widely adopted by the crystallographic community'.

Vigorous correspondence within the Subcommittee resulted in a series of draft reports that gradually evolved toward general consensus. An intermediate report was presented orally at an Open Meeting of the Commission during the XIV International Congress of Crystallography in Perth 1987.

The abstract and list of authors of the 1989 report are available.

1992-1995

The International Organization for Standardization (ISO) has issued a document (ISO, 1993), hereafter referred to as Guide, with the purpose of establishing general rules for evaluating and expressing the uncertainty of the result of a measurement . Based on a recommendation of the Comité International des Poids et Mesures, the rules are intended to be applicable to a broad spectrum of measurements. A recent NIST Technical Note (Taylor & Kuyatt, 1993, hereafter referred to as Note) presents a succinct account of the Guide that is useful for the general reader*.

The urgent need for such generally applicable rules derives from the fact that a measurement result is now considered complete only when accompanied by a quantitative statement of its uncertainty (see, for example, Guide page 4 or Note page 13). This new policy concerning measurement results was foreshadowed in Notes for Authors (1983). Therefore, the procedures and nomenclature adopted in the Guide and the Note seemed likely to have an impact on IUCr publication practice since it is Union policy to maintain consistency in its publications with internationally recommended nomenclature standards. Accordingly, the IUCr Commission on Crystallographic Nomenclature established a Working Group, drawn from the membership of the Subcommittee on Statistical Descriptors (Schwarzenbach et al., 1989), that was charged with undertaking an examination of the Guide and the preparation of such recommendations as may be appropriate for presentation to the crystallographic community.

Following examination of the above documents, the Working Group has concluded that the Guide represents a major step towards a rational and unified treatment of the expression of uncertainty in measurement, applicable to an extremely wide range of situations and problems. It noted with satisfaction that the report of the Subcommittee on Statistical Descriptors (Schwarzenbach et al., 1989) was generally in very good agreement with the recommendations of the Guide. One definition in the section Definition of statistical terms of Schwarzenbach et al., 1989 had to be modified and seven new definitions were added; one recommendation had to be expanded and four additional recommendations were proposed for use in all IUCr publications. The most important concept introduced by the Guide is expressed by the term uncertainty.

The abstract and list of authors of the 1995 report are available.

* Copies of NIST Technical Note 1297 may be obtained from the NIST Calibration Program or from Dr. B.N. Taylor or Dr. C.E. Kuyatt, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.

Updated 18th Sept. 1996

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