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,