Determining Sample Size for a Control Chart

The essential tool in statistical quality control is the control chart. In spite of the apparent simplicity of the control chart, most engineers, production men, and inspectors find that its use calls for an entirely new point of view. The power of this technique lies in its ability to separate out assignable causes of quality variation. This makes possible the diagnosis and correction of many production troubles and often brings substantial improvements in product quality and reduction of spoilage and rework. The control chart tells when to leave a process alone and thus prevents unnecessarily frequent adjustments that tend to increase the variability of the process rather than to decrease it. It also permits better decisions on engineering tolerances and better comparisons between alternative designs and between alternative production methods