Editing Dimensional analysis (section)

=== Quantity equations ===
{{distinguish|Quantity theory of money}}

A '''quantity equation''', also sometimes called a '''complete equation''', is an equation that remains valid independently of the [[unit of measurement]] used when expressing the [[physical quantities]].<ref name ="nist" />

In contrast, in a ''numerical-value equation'', just the numerical values of the quantities occur, without units. Therefore, it is only valid when each numerical values is referenced to a specific unit.

For example, a quantity equation for [[displacement (geometry)|displacement]] {{math|''d''}} as [[speed]] {{math|''s''}} multiplied by time difference {{math|''t''}} would be:
: {{math|1=''d'' = ''s'' ''t''}}
for {{math|''s''}} = 5&nbsp;m/s, where {{math|''t''}} and {{math|''d''}} may be expressed in any units, [[conversion of units|converted]] if necessary.
In contrast, a corresponding numerical-value equation would be:
: {{math|1=''D'' = 5 ''T''}}
where {{math|''T''}} is the numeric value of {{math|''t''}} when expressed in seconds and {{math|''D''}} is the numeric value of {{math|''d''}} when expressed in metres.

Generally, the use of numerical-value equations is discouraged.<ref name ="nist">{{Cite book|url=https://physics.nist.gov/cuu/pdf/sp811.pdf|title=Guide for the Use of the International System of Units (SI): The Metric System|last=Thompson|first=Ambler|date=November 2009 |publisher=DIANE Publishing|isbn=9781437915594|language=en}}</ref>