We previously discussed calibration, which assures a gage's accuracy. Accuracy, however, is only one requirement for effective measurements. The other is precision. We introduced these terms in Chapter 3, which discusses a manufacturing process' ability to meet specifications. The mean of an accurate process is on the target, which is usually halfway between the specifications.
Precision is the opposite of variation, and it has the same meaning in discussions of gages.
Unlike a manufacturing target, a gage's target is the actual dimension of the specimen it is measuring. If a dimension is 12.0 microns, we want the gage to report 12.0 microns. Table 5-1 shows how gage is like a manufacturing process. An accurate gage will, on average, report the specimen's actual dimension. If the piece's dimension is slightly out of specification, the gage may pass it.