Statistical Quality Control Using Control Charts

Control charts are used to monitor a process for some quality characteristic such as e.g., thickness, weight, defective fractions, etc. The common elements of quality control charts are outlined below:

• The Y-axis (vertical axis)) is a value axis that is scaled according to the unit of the corresponding summary statistic (e.g., standard deviation, mean, etc.)
• The X-axis (horizontal axis) is categorical and represents sample ID, order, sequence, run time, site, etc.
• Each point represents a summary statistic based on computation results from subgroup measurements of a continuously varying quality characteristic.
• The center line displays the average of the summary statistic. The upper and lower control limit lines (UCL and LCL) display the range of expected variations in the summary statistic. Points above the UCL or below the LCL lines are out-of-control points, while points inside the UCL and LCL are in-control-points.