The general approach to on-line quality control is straightforward: We simply extract samples of a certain size from the ongoing production process. We then produce line charts of the variability in those samples, and consider their closeness to target specifications (CL). If a trend emerges in those lines, or if samples fall outside pre-specified limits (LCL, UCL), then we declare the process to be out of control and take action to find the cause of the problem.