When should percent difference be used

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When should percent difference be used


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Contents:

  1. Percent Error
  2. Percent Difference

Watch the video for definitions and examples of percent error and difference:
Percent Error and Percent Difference

Percent Error and Percent Difference

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What is Percent Error?

Percent errors tells you how big your errors are when you measure something in an experiment. Smaller values mean that you are close to the accepted or real value. For example, a 1% error means that you got very close to the accepted value, while 45% means that you were quite a long way off from the true value. Measurement errors are mostly unavoidable: equipment can be imprecise, hands can shake, or your instruments just might not have the capability to measure accurately. Percent error will let you know how badly these unavoidable errors affected your results.

Formula

The formula is:
PE = (|accepted value  experimental value| \ accepted value) x 100%.

Example question: The accepted distance to the moon is 238,855 miles.* You measure the distance as 249,200 miles. What is the percent error?
Solution:
Step 1: Insert your data into the formula:
PE = (|accepted value  experimental value| \ accepted value) x 100% =
((|238,855 miles  249,200|) \ 238,855 miles) x 100% =
Step 2: Solve:
(10345 \ 238,855 miles) x 100% =
0.0433 * 100% =
4.33%.

*Thats the average distance, but lets assume its the distance on the day youre taking the measurement!

Note: in some sciences, the absolute value sign is sometimes (but not always) omitted. You may want to refer to your textbook to see if the author is omitting the absolute value sign. If you arent sure, the most common form is with the absolute value sign.

Alternate Wording

Accepted value is sometimes called the true value or theoretical value, so you might see the formula written in slightly different ways:

  • PE = (|true value  experimental value| \ true value) x 100%.
  • PE = (|theoretical value  experimental value| \ theoretical value) x 100%.

All three versions of the formula mean the exact same thing  its just different wording.

Alternative Definition of Percent Error using Relative Error

The percentage error is sometimes reported as being 100% times the relative error. Be careful though, because there are actually two types of relative error: one for precision and one for accuracy (not sure of the difference between the two? See: Accuracy and Precision). The definition 100% times the relative error is only true if you are using the accuracy version of relative error:

  • REaccuracy = (Absolute error / True value) * 100%.

The definition does not work if youre using the RE for precision:

  • REprecision = absolute error / measurement being taken.

What is Percent Difference?

Percent difference is practically the same as percent error, only instead of one true value and one experimental value, you compare two experimental values. The formula is:

percent-difference-formula



Where:

  • E1 is the first experimental measurement.
  • E2 is the second experimental measurement.

Example question: You make two measurements in an experiment of 21 mL and 22 mL. What is the percent difference?

Solution:

percent-difference-solved


CITE THIS AS:
Stephanie Glen. "Percent Error / Percent Difference: Definition, Examples" From StatisticsHowTo.com: Elementary Statistics for the rest of us! https://www.statisticshowto.com/percent-error-difference/---------------------------------------------------------------------------

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