Actually, like it is, there is no number of planets property, there is a class of planet (solar system planet) together with a number of instances property. This might save us : we can have two item :
* solar system planet (old style definition) and
* solar system planet (new style definition)
 This might just put the problem in another place though :) although this might not change the correctness of statements like <solar system>  <has part> <solar system planet (old style)> :)


Pluto can still be an old style definition planet, and maybe <solar system planet>
 is a subclass of <solar system planet (old style)>

Thinking about it, classes are naturally a good way to deal with definitions.

2015-04-29 19:35 GMT+02:00 Markus Krötzsch <markus@semantic-mediawiki.org>:
Hi,

General case first: Many statements depend on time and have an end date (e.g., population numbers). The general approach there is to (1) have a qualifier that clarifies the restricted temporal validity and (2) make the current statement "preferred". So your idea with the ranks was a good starting point, but it should be "normal" and "preferred" instead of "deprecated" and "normal". And infovarius was also right in this sense to use a temporal quantifier. Note that more than one statement can be preferred if more than one is current (this could be relevant, e.g., for the classes that Pluto is/was an instance of).


However, this answer is only about the general pattern of dealing with things that changed over time, and the intended use of ranks in this case. Things might be different here. It's a special case in that it was not so much the world that changed but the definition, so the real question is what our property "number of planets" really means:

(1) "Number of planets at a given time (given as a qualifier), based on the definition of planet adopted at this time"
(2) "Number of planets according to the definition that was used when the property was introduced"
(3) "Number of planets according to the definition that is current right now"

(3) is problematic since it means that the meaning of the property would change over time, and statements that were true will become false. I would strongly discourage this. But both (1) and (2) are possible. If one wants to use (1) then *every* statement with this property must have some time qualifier -- otherwise it will not make any sense since one would not know which definition is meant.

In case (2), the number of planets of our solar system is 8, and nothing else. It has never been 9 *according to the definition of planet used by this property*. So if this interpretation is adopted, then the statement with value 9 should really at best be there in a deprecated form, not in a temporal form. It could make sense to keep a deprecated form to warn other users that this should not be reintroduced.

One could also add more options, e.g., one could have a qualifier that specifies the definition of planet that is used. This would be a bit like (1) but instead of time one would now always need to specify a definition, and the statements would not be temporal at all (the number would always remain 9 according to the old definition). One could still use "preferred" to mark the statement that is based on the most common definition.

The world is beautifully complicated, isn't it? I'll leave it to you experts to discuss what makes sense here here :-)

Best regards,

Markus


On 29.04.2015 18:05, Thomas Douillard wrote:
Hi, a small question about qualifiers and ranks.

It is well known that the number of planets changed in 2006. Or did it ?
Of course, Pluto is still here, it's just its status that changed. The
definition of planets changed in 2006.

This imply that (imho), the statement  "the number of planets in the
solar system in 9" should be deprecated. But infovarius did not agree
with me and changed the rank of the claim back to normal and put an end
date. I still think it should be deprecated, but it raise me a question:
How are we supposed (if we are) to express an information about a
deprecation ? Should we include something about the deprecation in the
sources ? Should we have a qualifier ''deprecation date'' ?

https://www.wikidata.org/wiki/Q17362350


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