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RE: FW: Precision in engineering calculations

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Don't give him any ideas.
-----Original Message-----
From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)]
Sent: Wednesday, January 25, 2006 1:05 PM
To: seaint(--nospam--at)
Subject: Re: FW: Precision in engineering calculations


Did you finish your last sentence? I expected it to read "And he doesn't round off to the nearest whole dollar, unless it's in his favor."


Honles, Thomas wrote:
Slow down there... you're mixing apples and oranges here.

Accounting figures and engineering figures are two entirely different types of numbers.

Even in a multimillion dollar large corporation, it would be 'material' if an employee were to pocket a couple of $100 on a Friday afternoon from the till so he could rush off to Vegas and try to make a killing at the slots. Monday morning, security may be walking him right out of that corporation's side door and into a police cruiser for booking. The customer who wants the $0.99 hamburger most likely will not pay $1.00 even though its only 1% over.

Accounting figures vary in precision depending on the purpose. A year-end stockholder statement may be rounded to the nearest one-tenth of a million dollars, and the tax statement may be on the rounded whole dollar, but the individual employee gets a W-2 listing it accurate to the penny. The underlying assumption is that transactions carried out between individuals in our system of finances is based on an accuracy of one-hundredth of a dollar. Hence, the precision is flexible depending on use, but it must be accurate. Precision leads accuracy in accounting.

Engineering figures must be accurate as well, but the underlying assumption is that the degree of precision of any measurement is perhaps only 4 significant digits (within 0.1% of being 'true'). Hence the precision is fixed, or limited to the ability to physically measure something. Typically, materials in construction exhibit 5% to 10% variance from spec, so the number of significant digits at 3 is adequate. What does matter is the magnitude of those 3 digits: Is it 3.06 x 10^3 or 3.06 x 10^6? Is it 3,060 or 3,060,000? The correct order of magnitude leads to accuracy. Accuracy leads precision in engineering.

I know you probably all knew this, but it felt good to write it down again. I recently went through a similar train of thought explaining scientific notation to my 8th grader. And he doesn't round off to the nearest whole dollar on his allowance.

Best regards,

Thomas Honles, SE, PE
Los Angeles, CA

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