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# Significant (and insignificant) figures

last edited by 9 years, 4 months ago

The issue of significant figures is one that arises from the fact that we are not just doing math when you use numbers and symbols in science, but we are representing things in the physical world. To see how this works, consider the following problem in arithmetic:

1.843 x 3.686 = ?

If you were doing this in your math class, it would be easy. You would plug the numbers into your calculator and the result would come out.

1.843 x 3.686 = 6.793298

This is an exact result and you would get full credit in your arithmetic class. Indeed, if you gave anything else but this, you would probably get points taken off!

The problem is, that's not what you should do in a science class.

The problem that I gave was taken from a question that was taken from a physical problem. Suppose I am making a rectangular box to exactly fit two pennies into it side by side, how much area  do I need? This is shown in the figure below. (Source, Wikipedia Commons: Measurement)

Reading off the width of a penny in centimeters, I decided it was more than 1.84 cm, but not 1.85 cm, so I called it 1.843. The width of two pennies would then be twice that, and the area would be the product I gave you to calculate.

But it "really" could be 1.841 or even 1.839. I wouldn't bet against that.  If it were 1.841 cm, then my multiplication would actually be

1.841 x 3.682 =6.778562.

This is not the same as what I got in my first calculation -- 6.793298.  This tells me that the last four digits -- the "3298" -- are totally bogus. I have no idea whether they are correct or not. They could be anything.  Even the third place is a bit uncertain -- I got "9" the first time and "7" the next.  So I really shouldn't cite the result with more than three digits. Those are the only significant figures. The rest are insignificant figures.

You will sometimes find "automatic" rules for significant figures -- how many to keep. They can be convenient for those times when your brain has stopped working. For other times use the rule of thumb:

Only quote figures that you are pretty sure of -- perhaps plus one more.

There are (at least) four reasons to understand and use significant figures in this class and in science in general:

• Quoting a number is a communication.  When you cite a number you are telling your listener that you really believe those numbers. If you don't, you are misleading your listener, a serious error.
• Much of what we do in science relies on careful measurement. No measurement is perfect, so citing a correct number of significant figures (plus an uncertainty range -- usually called an error bar even though it's not about mistakes) gives information about the character of the measurement.
• You will need to understand when results you read in the scientific literature are accurate or not. Sometimes results in public forums (the web, news media, etc.) are given inappropriate significance (e.g., political polls, results of medical studies, etc.).
• You will lose credit on exams if you quote results to inappropriately many significant figures!

Joe Redish 7/6/11