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Re: FW: Precision in engineering calculations

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I routinely drop two digits of precision every year on my taxes, and the IRS is fine with it. I only use whole dollars, and the IRS suggests doing so.

In fact, I've been known to simply ignore $50 in my accounting. My wife, who is controller of a medium sized corporation, has even done that with much larger figures, and has had audits which have upheld her "rounding". In the financial world, it's known as whether or not the amount is material. For a small firm like mine, everything over $100 is material. For a large corporation, several thousand dollars may not be material (though it often is).

To believe that you have as much accuracy as you have precision will lead you into dangerous waters. A caliper that reads in hundredths (that's 0.00001") won't do you any good if it's not zeroed, and your measurement won't be very repeatable if you're holding the gauge block in your hand while you take the measurement.

I would tend to side with Tim's professors in this one. Now, if someone could talk to my fundamentals of engineering professor about the 5 points I lost for placing the middle horizontal line of a capital "E" slightly below the center of the vertical line, rather than slightly above, I'm right there with you!

Jordan

Ray Pixley wrote:

Those teachers were (1) wrong to do that, (2) short sighted, (3) unprofessional, and (4) think a slide rule is a high tech bell and whistle device. But that's life when they have that much power over you.

Anyway, significant figures are an issue if you are looking for a number that is the difference between two numbers that are nearly the same. When subtraction is being used, it matters. (Maybe other operations also.) If you don't have enough significant figures, you get garbage. No amount of tap dancing justification will be able to overcome that deficiency.

Now, ask those so called "teachers" if they round their taxes up to the next thousand dollars, simply because it is "approximately" what is due. Ask if when they see a penny on the street, do they pick it up or consider it worthless? If not a penny then what about a dime? A quarter?



----Original Message Follows----
From: "Timothy Allison" <t_allison(--nospam--at)illinoisalumni.org>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Re: FW: Precision in engineering calculations
Date: Wed, 25 Jan 2006 09:08:16 -0800

We were taught significant figures (and how to use them) in school.  A
couple teachers would go so far as to deduct a point or two for using
too many sig figs.  In practice, I ignore the rules and take it out to a
"clean" number - your example of 12319.82 vs. 12300 would be 12320 to
me.

I do agree with your thought that calculators give a false sense of
accuracy.  If you take something simple like "25000 - 12000", you're
still entering five digits regardless of there possibly being as few as
two significant figures, so writing the result with five significant
figures is kind of a natural reaction.


-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Wednesday, January 25, 2006 10:53 AM
To: seaint(--nospam--at)seaint.org
Subject: Precision in engineering calculations

Do engineering schools still teach the concept of "significant digits"
in design calculations? I generally like to show results to 3
significant digits (e.g., 12,300 lbs vs 12,319.82 lbs); although I
consider the real accuracy probably closer to 2 digits for most design
work. But I find that young engineers often show results such as 123.47
ft-kips. There are occasions when more significant digits are
warranted, such as when subtracting two large numbers, e.g. 12,319 lbs
minus 11,898 lbs. But most of the time such accuracy in numbers is not
warranted. For manual calculations, I feel that it takes extra time to
enter and write those extra digits, and it implies a greater accuracy
than is reality.

When I started college in 1970, I used that ancient device called a
slide rule. By the time I graduated, I could afford one of those
new-fangled calculators. I think that having to use a slide-rule gave
me a better understanding of significant digits, whereas calculators and
computers give people a false sense of design accuracy.

Nevertheless, I once did some design work for nuclear power projects and
was told that if my results showed a calculated stress of 24.1 ksi vs an
allowable stress of 24 ksi, the member size would need to be increased.
But that was less about accuracy and more about fear that someone would
report that the design indicated that some members were "overstressed".


William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

Scott Maxwell wrote:

"And I agree with you on the whole knat's rear arguement...it is not
needed or even warranted frequently in the "structural world". And it
is many times not even useful in repeative situations...in steel design,
one usually saves more money by having the same beam over and over and
over again rather than getting the absolute lightest members possible.
Not to mention that the concrete actually used likely is not going to
have the exact f'c that you used in design nor the steel to have the
exact fy that you used. So, doing precise calculations to some 5th
decimal place (although it does somewhat depend on the units used) is
typically a waste of time. I learned that one back in school when my
concrete professor "yelled" at me for using two or more decimal places
in my homeworks and pointed out that if my capacity was within roughly
5% or less of demand, I was likely "good to go"."


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