formula for percent difference and percent error?

Answer 1

Formula for percent difference is this:

(x1 - x2) / x1 multiplied by 100 gives the percentage difference of x2 from x1, without implying that x1 is right. So, 98 differs by 2% from 100.

Percentage error means the difference of the measured value from the true value. The true value is derived from a large number of measurements and is agreed as true by a group of experts. Velocity of light, duration of a second, mass of a kiolgram etc. are examples of 'true' values. When you measure the velocity of light, you get a value which differs from the true value by a small percentage and that is error.

Answer 2

Check my article I created on Wikipedia about Percent Difference. http://en.wikipedia.org/wiki/Percent_difference
The article about Percent Error is currently labeled as "Approximation Error".

Answer 3

Percent error comes from a value you acquired for some measurement versus the actual value, the generally/theoretically accepted value (i.e. 9.8 m/s^2 for acceleration due to gravity, stuff like that):
[(your measured value - the actual value)/(the actual value)] x 100%

Percent difference comes from two of your own measurements for the same experiment performed twice:
[(first measured value - second measured value)/((first measured value + second measured value)/2)] x 100%
or, the difference of the two measured values divided by the average of the two measured values (times 100%).