Economic values used in selection indexes are generally assumed to have a linear relationship to trait values. To the primary breeder in the commercial poultry industry, the relationships are non-linear (stairstep pattern) for most of the critical traits. The economic value of a trait (and the saleability of his stock) is determined by the performance of his stock relative to the competition. The point at which there is a substantial change in the economic value is referred to as a "breakpoint". If his stock is at the bottom of the breakpoint for a critical trait, he must quickly improve the trait or he will soon be out of business. If the trait is at the top of the breakpoint with a reasonable margin for error, selection pressure used to further improve the trait will be wasted.
As a consequence of breakpoints and the difficulty of translating commercial production values to pure line breeding values, the efficacy of including economic weightings in selection indexes is dubious. Restrictive selection indexes offer the best solution for the breeder. Methods developed for handling threshold traits 

F. T Schultz

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume X. Breeding programs for swine, poultry, and fish., , 215–227, 1986
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