Proposal: treatment of SU in DDLM dictionaries
There is some residual ambiguity around the treatment of su in our
DDLm dictionaries. Currently, if
_type.purpose for a data name
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:
Option (1) presupposes CIF format. DDLm should be agnostic regarding format
_suform of the data name be explicitly defined in the dictionary?
Is it legal to provide both the
_suform and the parenthetical form for a data name?
Does the value of a
Measuranddata name for the purpose of dREL include the SU?
_susuffix be a requirement when the current DDLm dictionaries contain data names that do not follow this? (e.g.
The following proposal aims to clarify these questions.
Measuranddata names have a corresponding data name for their SU explicitly defined;
That the convention for IUCr dictionaries is that this data name is formed by adding
_suto the original data name;
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;
That the definition for
Measurandis 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:
The new definition is format-agnostic
_suforms should be defined in the dictionary. Using
_suas 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.
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
*_sudata 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.
No, the value of a
Measuranddata name includes the main value only.
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.
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
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.