"It is still surprising how many people, from engineers to managers to quality professionals to technicians, possess limited understanding of product and process (manufacturing) limits.Claro que isto também dá para alguma batota.
These limits, applied to numerical measurements of key product quality characteristics, often drive behavior and actions as well as frustration for users and decision-makers. Operations can become dysfunctional when this lack of understanding produces further tightening of limits.
The differences between specification limits and control limits have been widely discussed since Walter Shewhart of Western Electric invented the process control chart in the mid-1920s. However, these differences seem to be poorly understood at all levels at many organizations and even among some quality professionals.
Let’s define specification limits. The general definition is limits within which a product would be expected to perform its stated and intended function for customer use. Specification limits, therefore, are related to product design. They should be set in the product design phase and effectively fixed for manufacture.
It might be surprising that some but not all organizations recognize this definition of specification limits. In one such organization the practice was to apply specification limits to averages of process results, thereby failing to acknowledge the application of specification limits to individual product units. Imagine the surprise when the new quality manager asked for the latest Cpk results and noticed Cpk values less than one!"
Recuar a 2008 e recordar "SPC (parte III) - Cartas de controlo e especificações a combinação contra-natura (a explicação)"
Trechos recuados de "The Role of Specification Limits"