Actually I think that having "no value" for the end date qualifier probably means that it has not ended yet. There is no other way to express whether this information is currently merely incomplete (i.e. it has ended, but no one bothered to fill it in) or not (i.e. it has not ended yet). This is pretty much the same use case as for normal claims.

Other qualifiers I could imagine where an explicit "no value" would make sense is P678, I guess.

In references it might make sense to state explicitly that the source does not have an issue number or an ISSN, etc., in order for example to allow cleanup of references and to mark the cases where a reference does not have a given value from those cases where it is merely incomplete.

I don't have superstrong arguments as you see (I would have much stronger arguments for "unknown value"), but I would prefer not to forbid "no value" in those cases explicitly, because it might be useful and it is already there.

[1] https://www.wikidata.org/wiki/Special:WhatLinksHere/Q18615010

On Thu, Apr 23, 2015 at 1:27 PM, Stas Malyshev <smalyshev@wikimedia.org> wrote:

Hi!

I was lately looking into the use of novalue in wikidata, specifically
in qualifiers and references. While use of novalue in property values is
pretty clear for me, not sure it is as useful in qualifiers and refs.

Example:

https://www.wikidata.org/wiki/Q62#P6

As we can see, Edwin Mah Lee is the mayor of San Francisco, with end
date set to "novalue". I wonder how useful is this - most entries like
this just omit end date, and if we query this in SPARQL, for example, we
would do something like "FILTER NOT EXISTS (?statement q:P582
?enddate)". Inconsistently having novalues there makes it harder to
process both visually (instead of just looking for one having no end
date we need to look for either no end date or end date with specific
"novalue") and automatically. And in overwhelming majority of cases I
feel "novalue" and absence of value model exactly the same fact - it is
a current event, etc. Is there any useful case for using novalue there?

Another example: https://www.wikidata.org/wiki/Q2866#P569

Here we have reference with "stated in":"no value". I don't think I
understand what it means - not stated anywhere? How would we know to
make such claim? Is a lie? Why would we keep confirmed lies in the data?
Does not have confirmed source that we know of? Many things do, why
would we have "stated in" in this particular case?
Summarily, it is unclear for me that novalue in references is ever useful.

To quantify this, we do not have a lot of such things: on the partial
dump I'm working with for WDQS (which contains at least half of the DB)
there are 14 novalue refs and 13 properties using novalue as qualifier,
leader being P582 with 200+ uses, and overall 422 uses. So volume-wise
it's not a big deal but I'd like to figure out what's the right thing to
do here and establish some guidelines.

Thanks,
--
Stas Malyshev
smalyshev@wikimedia.org

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