it is probably necessary to store the number of significant decimals.
Yes, that *is* the accuracy value i mean.
Daniel, please use correct terms. Accuracy is a defined concept and although by convention it may be roughly expressed by using the number of significant figures, that is not the same concept. Without additional information you cannot infer backwards whether usage of significant figures expresses accuracy or precision. See http://en.wikipedia.org/wiki/Accuracy_and_precision
Ok, there's some terminology confusion here. I'm using "accuracy" to refer to the accuracy of measurement (e.g. standard deviation), and "precision" to refer to the precision of presentation (e.g. significant digits). We need these two things at least, and words for them. I don't care much which words we use.
I do. And I think it is important for WIkidata to precisely express what it wants to achieve.
Accuracy has nothing to do with s.d., which is a measure of dispersion. You can have an accuracy of +/- 10 measured with a precision of +/- 0.1 (and a standard deviation for the population of objects that you have measured of 2).
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So 4.10 means that the last digit is significant, i.e. the best estimate is at least between 4.095 and 4.105 (but it may be better). . 4.10 +/- 0.005 means it is precisely 4.095 and 4.105, as opposed to 4.10 +/- 0.004, 4.10 +/- 0.003, 4.10 +/- 0.002 etc.
Yes, all this should be handled by the component responsible for parsing user input for quantity values.
But it cannot be because you have lost the information. I don't know whether +/- 0.005 indicates significant figures/digits or whether is is an exact precision_or_accuracy interval.
I think this may become clearer if you consider a value entered in inches:
1.20 inches. you convert: 1.20 +/- 0.05 in = 3.048 10^-2 m +/- 1.27 10^-3 m
If this is the only information stored, I have no information left whether I should display 3.048 or 3.0480 and whether the information +/- 1.27 10^-3 m is meaningful (no) or an artifact of conversion (yes).
It can be stored as an auxilliary data point, that is, as a qualifier ("measured in feet"). It should not IMHO be part of the data value as such, because that would make it extremely hard to use the values in a database.
You are correct insofar that I propose you need to store two units: the normalized one (SI units only, and no prefix - and even though the SI base unit is kg I would store gram) and the original one plus the original unit prefix.
If you do that, you can store the value in a single normalized unit, provided you back-convert it prior to display in Wikidata.
I don't think the original unit is a meaningless qualifier, it is vital information for context.
Gregor