Relative change

Percent Difference is the numerical interpretation of comparing two values with one another. The general requirement for selecting two values to be compared is that the user of this technique expects the two values to be numerically equivalent. In other words, obtaining a percent difference of 0% is the optimum result as it explains that the two values are exactly the same. Not a general requirement, but common use shows that the two values usually will pertain to the same object (let's say the mass of an object, a material's characteristic, or maybe the discharging time of a capacitor), but each value will be calculated using two different methods and/or theories. Emphasis must be made on the word calculated, because the most important requirement for the two values that are being compared using Percent Difference is that they had needed to be calculated indirectly of measurement of the objective value. In other words, neither of the two values can be the actual or accepted value of the objective value.

The two values are determined using theories and measurements that are to be tested with respect to an accepted value by the scientific community. Example is someone measuring the length and period of a pendulum to determine the acceleration of gravity, but must relate the value to the accepted value of gravity by the scientific community. This is starting to sound like another comparison technique called Percent Error whenever one determines an experimental value and is comparing it to the accepted or actual value. But Percent Difference is different. Neither of the two values are the accepted or actual value, they are both an experimental value determined by two different techniques but describing the same objective value.

Formulae

The equation for determining the Percent Difference by comparing values x₁ and x₂ is:

$\%Diff={\cfrac {|x_{1}-x_{2}|}{\left({\cfrac {x_{1}+x_{2}}{2}}\right)}}\times 100$

In sentence form, one is dividing the absolute difference of the two values by the average value of x₁ and x₂. Because this equation contains the absolute function, percent difference will always be positive and therefore it does not matter which value one assigns to the "dummy" variables (x₁ and x₂) used in these equations shown in this article. An easier form of the equation can be calculated as,

$\%Diff={\cfrac {2\times |x_{1}-x_{2}|}{x_{1}+x_{2}}}\times 100$

It is important to note that both values (x₁ and x₂) must contain the same units in order to be compared correctly with one another. And as mentioned before, a zero percent difference is optimum and the higher the percent value, the less precision of the two values.

Lastly, percent difference is unitless; however, in the two forms given above, the calculated value must be quoted as a percentage. One may quote the difference without using percent by not multiplying the fraction by 100,

$Diff={\cfrac {|x_{1}-x_{2}|}{\left({\cfrac {x_{1}+x_{2}}{2}}\right)}}$

Final note to make is that a lot of confusion lies is mistakenly assuming that percent difference is the same as percent error. The difference is that percent difference is comparing two experimental values whereas percent error compares one experimental value with the actual/accepted value.

References

"Understanding Graphing and Measurement" (PDF). NCSU. Retrieved 2007-03-27. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)