Property talk:P8111

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recommended unit of measurement
unit in which a quantity is measured as recommended by a standard (SI, ISO, IEC, etc.)

Represents	unit of measurement (Q47574)
Data type	Item
Domain	individual quantity (Q71550118)
Example	mass (Q11423) → kilogram (Q11570) area (Q11500) → square metre (Q25343) speed (Q3711325) → kilometre per hour (Q180154)
Tracking: usage	Category:Pages using Wikidata property P8111 (Q106835997)
See also	measured physical quantity (P111), measures (P2575), measurement scale (P1880), ISQ dimension (P4020)
Lists	Items with the most statements of this property Count of items by number of statements (chart) Items with the most statements Items with the fewest statements Count of items by number of sitelinks (chart) Items with the most sitelinks Items with the most sitelinks, but no link to one Wikipedia (sample: Welsh Wikipedia, cywiki) Items with the fewest sitelinks Most linked wikis Items with the most identifier properties By value of property Most statements Fewest statements Most sitelinks Most linked wikis Total of sitelinks on items Most identifiers Fewest identifiers Items with no other statements Most recently created items Items with novalue claims Items with unknown value claims Usage history (total) Usage history (main statement) Chart by item creation date Database reports/Constraint violations/P8111 Map Random list
Proposal discussion	Proposal discussion
Current uses	Total 1,312 Main statement 1,298 98.9% of uses Qualifier 14 1.1% of uses
Search for values

[create Create a translatable help page (preferably in English) for this property to be included here]

Type “individual quantity (Q71550118)”: item must contain property “instance of (P31), subclass of (P279)” with classes “individual quantity (Q71550118)” or their subclasses (defined using subclass of (P279)). (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P8111#Type Q71550118, SPARQL

Value type “SI or accepted non-SI unit (Q87252761), counting unit (Q78754808)”: This property should use items as value that contain property “instance of (P31)”. On these, the value for instance of (P31) should be an item that uses subclass of (P279) with value SI or accepted non-SI unit (Q87252761), counting unit (Q78754808) (or a subclass thereof). (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P8111#Value type Q87252761, Q78754808, SPARQL

Citation needed: the property must have at least one reference (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P8111#citation needed

Property “measured physical quantity (P111)” declared by target items of “recommended unit of measurement (P8111)”: If [item A] has this property with value [item B], [item B] is required to have property “measured physical quantity (P111)”. (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P8111#Target required claim P111, SPARQL, SPARQL (by value)

Without qualifiers: this property should be used without any qualifiers. (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P8111#allowed qualifiers

Allowed entity types are Wikibase item (Q29934200): the property may only be used on a certain entity type (Help)

Exceptions are possible as rare values may exist. Exceptions can be specified using exception to constraint (P2303).
List of violations of this constraint: Database reports/Constraint violations/P8111#Entity types

System of units

@Toni 001: I propose to add applies to part (P518) with system of units for this unit (now mostly International System of Units (Q12457)). --Infovarius (talk) 17:17, 26 April 2020 (UTC)

@Infovarius: The SI is the recommended unit system. In the long proposal process we reached the conclusion that this property should be narrowly scoped and only have a single best rank and sourced unit. It would be against the spirit of this property to also list units of other unit systems like cgs, dimensionless units etc. Toni 001 (talk) 18:19, 26 April 2020 (UTC)

The following query demonstrates how to find SI units that measure length:

select distinct ?unit ?unitLabel where {
  ?unit wdt:P111 wd:Q36253 .  # a physical quantity
  ?unit wdt:P31 / wdt:P279 wd:Q61610698 .  # a class of units
  service wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en" . }
}

Try it!

To adapt this to other unit systems we could create items for "EMU unit", "ESU unit", ... and then link units to them using instance of (P31).

This is just one possible way to model unit systems. Others include proposing a new property "system of units", and yet another to use part of (P361) (on the unit items).

What do others think? How should we model "systems of units"? Toni 001 (talk) 09:08, 28 April 2020 (UTC)

1) There are different recommended unit systems in different fields.

2) The proposal discusssion is tl;dr for me (may be I'll invest some day a couple hours for this) but I see that "single best rank" was said about preffixed units.

3) Not more difficult to create a query for SI units in my model too.

4) I see no need in dedicated properties for each system of units, this is enough. --Infovarius (talk) 00:52, 3 May 2020 (UTC)

About 1: This might need clarification. While it is true (and totally fine) that in certain contexts one uses more useful units, those are not recommended in the word-sense used here. This property intends to list units recommended by a standards body.

About 3: Not all the recommended units can be traced to the SI (see the proposal for an example), but they are still recommended by a standards body. If one were to allow multiple values qualified by unit system one would loose the ability to query for the recommended unit, because a query like "recommended units for unit system = SI" would miss those. The reason is simply that there is no single unit system that contains all the recommended units.

Finally, I don't see why the unit system needs to be mentioned in this property. The unit system, which a unit is part of, is a property of the unit, and therefore should be stated there - on the unit item - in one form or the other. Toni 001 (talk) 04:41, 4 May 2020 (UTC)

They were recommended at some moment: "In 1873, a committee of the British Association for the Advancement of Science, including physicists James Clerk Maxwell and William Thomson recommended the general adoption of centimetre, gram and second as fundamental units...". So they fit into this property. With appropriate rank and qualifier. Infovarius (talk) 02:35, 15 May 2020 (UTC)

That's not a standard. As mentioned above, the word recommended is to be understood as "prescribed by a standard". (Note that standards bodies usually prefer the word "recommend" instead of "prescribe" - see for instance the W3C which produces recommendations.) Toni 001 (talk) 07:17, 15 May 2020 (UTC)

Isn't CGS a standard itself? Is this property a overall campaign against popular non-SI units?? They were gladly linked before the creation of this property... --Infovarius (talk) 00:36, 17 May 2020 (UTC)

CGS is a unit system, not a standard. After listing the base units of any unit system, it is a mechanical task to list all units of that system (plus a few named combinations, like, say, poise). Simply listing such (trivial) combinations does not add much value. Please review the (lengthy) proposal discussion and history: Initially this property had a wider scope which would have allowed listing basically any unit, but in the process the scope was reduced because simply having an inverse of P111 would not add much (or any) value. (Note also that this property is not limited to SI; the discussion contains an example.) Toni 001 (talk) 11:57, 17 May 2020 (UTC)

Single best value constraint

There are are examples where the standard recommends more than one unit. Initially I had though that we could simply make the first one that's mentioned the preferred one, but now I don't think that we should add such interpretation (as it does not follow from the standard). A frequent example is the recommendation to use either rad/s or s⁻¹. Therefore I removed the "single best value constraint". Toni 001 (talk) 09:19, 27 July 2020 (UTC)

• @Toni 001: Might these better serve as constraint exceptions (if there aren't many)? --SilentSpike (talk) 11:36, 27 July 2020 (UTC)

@SilentSpike: I've been entering a few cases recently and more are coming. That's why I started thinking about what the original justification was. One motivation was to have a unique target for unit conversions. But given that the standard organization can't make up their mind (those are tough questions involving actual use or more fundamental question like the dimensionlessness of angles - so no criticism here), we should not pretend to have the right answer neither. I think that the right choice depends on the context which could in theory be modeled somehow in the future (though I don't have a suggestion at the moment - it also seems like a lower-priority question). Toni 001 (talk) 09:22, 28 July 2020 (UTC)