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Proposal: treatment of SU in DDLM dictionaries

Introduction

There is some residual ambiguity around the treatment of su in our DDLm dictionaries. Currently, if _type.purpose for a data name is Measurand, the DDLm attribute dictionary states:

       Used to type an item with a numerically estimated value
       that has been recorded by measurement or derivation. This
       value must be accompanied by its standard uncertainty
       (SU) value, expressed either as:
         1) appended integers, in parentheses (), at the
            precision of the trailing digits,       or
         2) a separately defined item with the same name as the
            measurand item but with an additional suffix '_su'.

This raises the following issues:

  1. Option (1) presupposes CIF format. DDLm should be agnostic regarding format

  2. Should the _su form of the data name be explicitly defined in the dictionary?

  3. Is it legal to provide both the _su form and the parenthetical form for a data name?

  4. Does the value of a Measurand data name for the purpose of dREL include the SU?

  5. Can the _su suffix be a requirement when the current DDLm dictionaries contain data names that do not follow this? (e.g. _refln.F_sigma).

The following proposal aims to clarify these questions.

Proposal

  1. That all Measurand data names have a corresponding data name for their SU explicitly defined;

  2. That the convention for IUCr dictionaries is that this data name is formed by adding _su to the original data name;

  3. That the parenthetical form of presentation of the su value for CIF syntax is understood as a shorthand assignment of this su value to the associated SU dataname;

  4. That the definition for Measurand is therefore rewritten as:

       Used to type an item with a numerically estimated value
       that has been recorded by measurement or derivation. A 
       data name definition for the standard uncertainty (SU) 
       of this item must be provided in a separate definition
       with `_type.purpose` of `SU`.

The above questions are then answered as follows:

  1. The new definition is format-agnostic

  2. Yes, _su forms should be defined in the dictionary. Using _su as a suffix is purely an IUCr convention which is not always followed (e.g. _refln.F_sigma) and therefore not appropriate for the DDLm attribute dictionary to specify.

  3. Yes, it is syntactically legal to have both forms, as the CIF syntax can have no embedded understanding of the meaning of the data names, including *_su data names, and therefore duplication cannot be detected as a syntax error. It is instead a semantic error in the same way as a cell volume - cell parameter mismatch would be. Thus if the two values provided agree, there is no error, and if they disagree, the software can take steps based on the importance of the mismatch to the particular computation.

  4. No, the value of a Measurand data name includes the main value only.

Discussion

In order for DDLm to be format-agnostic, each format needs to associate some location in that format with a data name. The appearance of a value in a CIF file without the data name appearing as well (as is being proposed above) is thus not unusual in general, simply for CIF this association is usually transparent due to the data name appearing in the format itself.

Compatibility

CIF authoring software

Authoring software remains free to append SU in parentheses.

CIF reading software

Legacy CIF reading software will have the same problems that it presumably has with the new 'dotted' data names, in the sense that a data name that was unknown at the time of software preparation has been used to provide a value. This is a cost that we have accepted. It seems unlikely that

Other comments

The su of a data item must always have been treated separately in software, as software must handle the su differently to the main value due at least to the differences in the way errors are propagated. The creation of a separate data name captures this fact.