In everyday language, we use the terms accuracy and precision interchangeably. In mathematical terminology, however, the accuracy of a measure (an approximate number) is defined as the ratio of the size of the maximum possible error to the size of the number.
This ratio is called the relative error. We express the accuracy as a percent, by converting the relative error to a decimal and subtracting it from 1 (and writing the resulting decimal as a percent). The smaller the relative error, the more accurate the measure.
This interactive activity illustrates how it works. We can measure the length and the weight of two different items: a notepad and a desk. Although we measure each one to the same level of precision, the relative error differs because of the size of the object. Note 13
This activity requires the Flash plug-in, which you can download for free from Macromedia's Web site. A non-interactive version of this activity is also available.