Fractional distillation is a technique that separates the various constituents in a mixture utilizing the differential boiling points(BP) of the individual constituents. As the mixture is heated, low BP constituent is removed from the top of the distillation column while the high BP constituent is removed from the bottom.
Very similarly, the software under development has a “mixture of various types of defects”. Our job is remove the each specific instance of the various defect types from the software. Akin to how we see a fractional distillation column consisting of multiple filtration levels, visualize test cases as a column segregated into various quality levels, where each quality level denotes a point where certain types of defects(aka constituents) are removed. Quality levels are like milestone markers in the final destination of software-under-development to the "good-enough quality" destination.
A clear organization of test cases by specific quality level allows us be purposeful and enables us to see the quality growth of the system under test. It allows to optimize test effort by ensuring that we minimize back-tracking.
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