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Why Do We Round?

Date: 12/18/2002 at 16:08:59
From: Sara
Subject: Rounding

Why do we round?
What's the whole point of rounding?

I think we round to get an estimate of something.


Date: 12/18/2002 at 23:15:12
From: Doctor Peterson
Subject: Re: Rounding

Hi, Sara.

Here are a couple of discussions I found in the Dr. Math archives:

   Why Estimate?
   http://mathforum.org/library/drmath/view/61226.html 

   Why Round Decimals?
   http://mathforum.org/library/drmath/view/58940.html 

I would say that we round for a couple of reasons:

One is to estimate, as you suggest: if we have more or less exact 
numbers, but only need an approximate product, say, then we can save 
work by rounding the numbers before we do our calculations. This 
implies that our data are more accurate than we need (at least at the 
moment).

Another is to reduce the accuracy of a result to reflect the accuracy 
of the data. You can read about this by searching for "significant 
digits." For example, if we measure the sides of a rectangle as 3.1 
and 5.7 inches, then the area can be calculated as 17.67 square 
inches. But with only two digits of precision in the lengths, we can 
only be sure of two digits in the answer, so we should round the 
answer to 18 square inches to show how precise our numbers are.

In each case we are reducing the precision of our numbers for some 
reason. 

If you have any further questions, feel free to write back.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/ 
Associated Topics:
Elementary Number Sense/About Numbers
Middle School Number Sense/About Numbers

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