1. A method of determining whether or not to perform an action based at least partly on an estimated maximum test range, the method comprising: attaining results generated from a parametric test performed on semiconductor devices included in a control set comprising a subset of a population of semiconductor devices; selecting from among said semiconductor devices at least one extreme subset including at least one of a high-scoring subset including all devices whose results exceed a high cut-off point and a low-scoring subset including all devices whose results fall below a low cut-off point; plotting at least results of said at least one extreme subset as a normal probability plot located between a zero probability axis and a one probability axis ; fitting a plurality of curves to a plurality of subsets of said results of said at least one extreme subset respectively; extending each of said plurality of curves to said zero probability axis for said low-scoring subset or to said one probability axis for said high scoring subset thereby to define a corresponding plurality of intersection points along said zero or one probability axis; defining an estimated maximum test range based on at least one of said intersection points; and determining whether or not to perform an action based at least partly on said estimated maximum test range.