Hi Tom,
Thanks for your message, you are touching upon a very crucial point, the
justification of completeness. For several properties, completeness is
clearly impossible ("SignificantEvent", "Occupation", "AwardsReceived" -
what is significant, what counts as an occupation, and didn't everyone once
win something like the 2nd place in the school chess tournament?).
Still, for some properties, we believe stating completeness to be
justified, or at least no more unjustified than entering data. Are Malia
and Sasha all children of Obama? To the best of common knowledge yes, and
if we are to doubt this, we might similarly doubt whether Malia is a child
of Obama at all, so stating completeness looks to me no less justified than
stating that Malia is a child of Obama.
Regarding the other aspect you mention, the time variance of data, the best
we can think of is to qualify completeness statements with an "as-of"
timestamp.
"Malia and Sasha are all children of Obama, as of 2nd of March, 2016".
Besides that there may be a portion of well-known history that is not going
to change any more, such as "founders of Apple", "Kings of the Italian
Monarchy", "Participant nations in the 2012 Olympics" and similar, where
time does not play a role.
Deciding and agreeing for which properties completeness assertions make
sense and for which not would probably require a significant effort by the
domain experts (i.e. you, the Wikidata authors), the above examples are
only guessed.
Do you know potentially of any guidelines/agreements in Wikidata to decide
which properties ("child") are more well defined, and which ones
("significantEvent") are not?
Thanks and best wishes,
Simon