Percent Difference

One uses percent difference when comparing two calculated or experimental values to each other. Typically one is interested in the percent difference of two values pertaining to the same property or characteristic of an object or system (mass, velocity, charge, etc). Typically, both values are calculated using different methods, theories, or devices. Just as with percent error, calculating percent difference is quite easy.

Percent Error

Applied when comparing an experimental value with a theoretical value, which is considered the correct value (sometimes called the litature value)

Percent Difference

Applied when comparing two experimental quantities, measured 1 and measured 2, neither of which can be considered the "correct" value.

Percent Error

and

Percent Difference

Sometimes scientists will want to compare their results with those of others, or with a theoretically derived prediction.  Each of the these types of comparisons call for a different type of analysis, percent difference, and percent error respectively

Important Note:

Do not believe that your value for percent error or percent difference is a measure of how well you performed the experiment.   To be honest percent error and percent difference is nothing more then telling you how close did you get from expected value (or in the case of percent difference how close you are compared to everyone else).   It's only when you consider your total percent uncertainty with your percent error / percent difference do we get a better understanding of how well you have performed your experiment.