Percent Error and Percent Difference

Percent Error

When comparing an experimental result to a value determined by theory or to an accepted known value (like

g = 9.8 m/s²)

we determine the difference between the experimental value and the theoretical value as a percentage of the theoretical value. In the definition below, "theoretical" is the value that is determined from theory (i.e., calculated from physics equations) or taken as a known or accepted value like g.

( 1 )

% error =

|theoretical − experimental|

theoretical

× 100%

Percent Difference

When wanting to compare two values that are both determined by experimentation, we don't have a "correct" or accepted value to which to compare, so we simply look at how different the two values are as a percentage of their average value:

( 2 )

% difference =

|value 1 − value 2|

value 1 + value 2

× 100%.