Eccentricity is the relationship of the geometric center of a track to the geometric center of the disc spindle hole. The standard for CD-ROM is ±50 microns.
A common method used in CD analyzers to measure eccentricity is to use the optical head focused on the disc to count track crossings. The servo control mechanism that provides the tracking for the head is opened, so that as the disc spins, the laser beam remains focused on the information, but does not follow the tracks. To whatever degree the disc is eccentric, the result will be that the tracks move back and forth across the laser beam, which itself remains stationary. By counting the number of track crossings, eccentricity can be measured.
The problem, of course, is that any system used to measure eccentricity is itself subject to the inaccuracies contributed by the mounting system of the measuring device. While elaborate fabrication methods can reduce this inherent eccentricity, nonetheless, it remains. Moreover, one cannot simply subtract the device eccentricity contribution from the total, as there is no guarantee that the direction of the eccentricity of the measuring device is the same as that of the CD.