A process capability index is based on specification limits,also named as tolerances. We assume two-sided specification limits defined by the lower specification limit (LSL) and the
upper specification limit (USL), and which generally indicate ranges of acceptance quality characteristics. In other words,a product is considered as acceptable if its characteristics are
within the specification interval [LSL, USL]. For example, the specification limits for the volume of bottles may be specified as 2 liters ±0.05 liters, which indicates that LSL ¼ 1:95
liters and USL ¼ 2:05 liters. One-sided specification limits can be also defined. For example, the volume of bottles may have the lower specification limit LSL ¼ 1:95, but not an upper specification limit (see also Montgomery, 2009, p. 9).