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).