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Relative Distance Inside Spec
The relative distance inside spec is the relative distance that the interval for values is from the specification limits. Its formula is:
where USL is the upper specification limit, LSL is the lower specification limit, Target is the target, UT is the upper end of the interval for values and LT is the lower end of the interval for values. It represents the ratio of the distance from the interval for values to the nearest spec and half the width of the spec.
The relative distance inside spec is unitless and is defined for all three types of tolerances, including worse-case tolerances. It requires that a target be specified for those outputs that have a 1-sided spec limit. Of all the optimization criteria, it is the only one that can be used to simultaneously optimize multiple output variables regardless of the method of tolerancing used.