Statistical Process Control SPC
Variance (fluctuation) is the arch-nemesis of quality; you must manage a process not by gut feelings, but mathematically within the boundaries of control limits.
At a microscopic level, no two manufactured parts are ever perfectly identical. Every process exhibits a certain degree of variation stemming from common (random) and special (unexpected) causes. Statistical Process Control (SPC) is a visual and analytical monitoring mechanism used to detect whether this variation is reaching dangerous levels while the products are still on the assembly line. Instead of measuring quality at the end of the line, it keeps the process under strict control right at its beating heart.
The most popular tools of SPC are Control Charts, which establish the process mean along with the acceptable Upper and Lower Control Limits (UCL and LCL). If the data points fluctuate randomly within these limits, the process is deemed "statistically in control." However, if points breach the limits or display a clear trend within them (such as 7 consecutive ascending points), it instantly signals that the system's equilibrium is compromised and immediate intervention is required.
