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Boundless Physics
The Basics of Physics
Significant Figures and Order of Magnitude
Physics Textbooks Boundless Physics The Basics of Physics Significant Figures and Order of Magnitude
Physics Textbooks Boundless Physics The Basics of Physics
Physics Textbooks Boundless Physics
Physics Textbooks
Physics
Concept Version 9
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Round-off Error

A round-off error is the difference between the calculated approximation of a number and its exact mathematical value.

Learning Objective

  • Explain the impact round-off errors may have on calculations, and how to reduce this impact


Key Points

    • When a sequence of calculations subject to rounding error is made, these errors can accumulate and lead to the misrepresentation of calculated values.
    • Increasing the number of digits allowed in a representation reduces the magnitude of possible round-off errors, but may not always be feasible, especially when doing manual calculations.
    • The degree to which numbers are rounded off is relative to the purpose of calculations and the actual value.

Term

  • approximation

    An imprecise solution or result that is adequate for a defined purpose.


Full Text

Round-off Error

A round-off error, also called a rounding error, is the difference between the calculated approximation of a number and its exact mathematical value. Numerical analysis specifically tries to estimate this error when using approximation equations, algorithms, or both, especially when using finitely many digits to represent real numbers. When a sequence of calculations subject to rounding errors is made, errors may accumulate, sometimes dominating the calculation.

Calculations rarely lead to whole numbers. As such, values are expressed in the form of a decimal with infinite digits. The more digits that are used, the more accurate the calculations will be upon completion. Using a slew of digits in multiple calculations, however, is often unfeasible if calculating by hand and can lead to much more human error when keeping track of so many digits. To make calculations much easier, the results are often 'rounded off' to the nearest few decimal places.

For example, the equation for finding the area of a circle is $A = \pi r^2$. The number $\pi$ (pi) has infinitely many digits, but can be truncated to a rounded representation of as 3.14159265359. However, for the convenience of performing calculations by hand, this number is typically rounded even further, to the nearest two decimal places, giving just 3.14. Though this technically decreases the accuracy of the calculations, the value derived is typically 'close enough' for most estimation purposes.

However, when doing a series of calculations, numbers are rounded off at each subsequent step. This leads to an accumulation of errors, and if profound enough, can misrepresent calculated values and lead to miscalculations and mistakes.

The following is an example of round-off error: 

$\sqrt{4.58^{2} + 3.28^{2}} = \sqrt{21.0+10.8}=5.64$ 

Rounding these numbers off to one decimal place or to the nearest whole number would change the answer to 5.7 and 6, respectively. The more rounding off that is done, the more errors are introduced.

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