"WillR" wrote in message
...
Arch wrote:
SNIP.....
Arch:

I must apologize - I forgot the following..

When multiplying or dividing -- the percentage of error adds absolutely,
when adding and subtracting, the absolute error adds absolutely.

That would make it clear why you have the rules about guard digits and
places of precision.

For example if you multiply two number together, (and you had been able
to make the measurements and obtain the two numbers to 5% accuracy...)
The resulting answer would be accurate to 10% -- so any digits beyond
the first decimal place would be guard digits or used for rounding...

So you could haver 5 decimal places of precision -- but the number would
be accurate to one decimal place...

So in this situation if you had 1.115 (plus as many digits more as you
please since they are not relevant) in the calculator window after the
calculation -- the _answer_ would be 1.1 and be accurate to _one_
decimal place... No matter what your calculator said...

All kidding aside -- you should remember this when doing calculations
when you multiply or divide measured numbers.


Adding or subtracting..
1 1/2 (+- 1/16 ) + 1 1/4 (+- 1/16) is 2 3/4 (+- 2/16) or

2 2/4 (+- 1/8 ) in other words... Because it is the ABSOLUTE SUM of the
two estimated errors that is the error factor.
SNIP .......
=================================
We all (or at least the veteran wood/ metal/ etc. workers) know that any
tolerances will accumulate in the direction that will totally defeat any
attempt at assembly until the entire project has been modified to the point
where it is no longer identifiable as the original concept showed it on the
plans (if any plans were actually drawn!) {:-)

Ken Moon
Webberville, TX