Often, laboratory experiments are designed to determine the value of well-known physical quantities such as the value of p or the acceleration due to earth's gravity, g. Since these quantities have accepted or true values, we can calculate the percent error between our measurement of the value and the accepted value with the formula

Sometimes, we will compare the results of two measurements of the same quantity. For instance, we may use two different methods to determine the speed of a rolling body. In this case, since there is not one accepted value for the speed of a rolling body, we will use the percent difference to determine the similarity of the measurements. This is found by dividing the absolute difference of the two measured values by their average, or