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Significant Figures and Rounding Rules

    "This web site is dedicated to the discussion of issues relating to the proper teaching of significant figures and rounding rules in high school and college science courses."  - Christopher Mulliss (page Author)


Rounding rules for arimethic operations are not perfect.  We therefor recommend that rounding rules are taught in a way that conveys the following information:
1. The fact that rounding rules are useful approximations but that they can fail.
2. The accuracy of the rounding rule.
3. The ways that a given rounding rule can fail (called failure modes).
4. How often each failure mode occurs.

In order to live up to our own standards, we have created tables that summarize our body of work and present educators and students alike with the information that they need to apply and fully understand the rounding rules that they use everyday.

  • Standard Rounding Rules    Table showing the "standard" rounding rules for many common operations: multiplication and division, addition and subtraction, common (base-10) and natural logarithms, and common (base-10) and natural exponential functions.

  • Recommended Rounding Rules    Table showing our recommended rounding rule for each operation.  The recommended rules were chosen based on their simplicity, accuracy, and "safety".  A "safe" rounding rule does not predict fewer significant digits than are justified because doing so causes valuable information to be discarded in the rounding process.

  • Higher-Accuracy Rounding Rules    Table showing the several higher-accuracy rounding rules. These rounding rules emphasis accuracy over simplicity.

  • The Work of Christopher Mulliss and Dr. Wei Lee’s Team1

        In 1996, I read a note in The Physics Teacher by R. H. Good.  In this note, Good describes a division problem that he discovered by accident in which the standard rounding rule caused precision to be lost in the result of the calculation.  In other words, the standard rounding rule caused a digit to be discarded even though we were justified in keeping it!  Thus, the "rule" that we all use is potentially dangerous to our data - an unacceptable situation!!

        This note caused me to question the standard rounding "rule" for multiplication and division.  Where did this rule comes from?  How often does it fail?  How can it fail?  Is there a better alternative?  How is the rule for multiplication and division related to the rule for addition and subtraction?  With the help of Dr. Wei Lee, I began a detailed investigation this so-called rule that resulted in the answers to these fundamental and important questions.  The results prove to be quite surprising!

    1 Department of Physics, Chung Yuan Christian University in Chung-Li, Taiwan

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    This page is maintained by Christopher Mulliss who is solely responsible for its contents.