1.1. AVERAGE AND VARIATION
First, no one knows how to make anything "perfect." If you order 50 1.000" diameter ball bearings and then measure the bearings once you get them, you will find that they are not exactly 1.000" in diameter. They may be extremely close to 1.000", but if you measure them carefully, with a very good calibrated measuring device, you will find that the bearings are not exactly 1.000".
The bearings will vary from the 1.000" target in two ways. First, the average diameter of these 50 bearings will not be exactly 1.000". Whatever amount the average deviates from the target 1.000" is due to the bearing manufacturing process being off-center. Second, there will be a spread of measurements around the average bearing diameter. ...
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