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Subject:RE: Tolerance in text From:"Jonathan West" <jwest -at- mvps -dot- org> To:"TECHWR-L" <techwr-l -at- lists -dot- techwr-l -dot- com> Date:Wed, 22 Feb 2006 10:39:32 -0000
The discussion on implied tolerances seems to be highlighting a common
misunderstanding regarding the difference between precision, accuracy and
tolerance.
When reporting a measured value, the precision is nothing more than the
number of significant digits included. (e.g 123 or 24.5)
When expressing a measured value, the precision used should not be greater
than the accuracy (i.e. the measurement uncertainty of the meaured value).
For instance, there is no point in stating a measured value of 24.00005 if
the measurement uncertainty is 0.03.
Where the accuracy is *not* explicitly stated, then the author should expect
the reader to assume that the accuracy is comparable to the precision. So,
where the accuracy is 0.03, there are two acceptable ways of expressing that
measured value. One is to state the accuracy explicitly, i.e. 24.0 +/- 0.03.
The other is to leave the accuracy implicit, simply by stating 24.0, in
which case the reader will infer an accuracy of +/-0.05. If the difference
in accuracy between 0.03 and 0.05 matters, then you have to state it
explicitly.
None of the above has anything to do with tolerances, which are concerned
with communicating a *requirement* not a measurement. A specification
requiring (as a trivial example) a "one-inch cube of pure iron" is
absolutely meaningless by itself. An engineer will need to know all sorts of
dimensional tolerances, a chemist will need to know the permitted level of
chemical impurities, and a physicist will need to know the allowable
isotopic composition, all of which may affect the properties of the cube of
iron in some significant way.
If single values are specified for properties that have a continuous range,
then the person doing the specifying is not doing his job properly. Anybody
reading such a specification really ought to query it and ask what the
tolerance is. Making assumptions about tolerances is asking for trouble.
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