Lower and Upper Specification Limits (LSL, USL)

In order to monitor industrial processes, control charts are often employed. They are graphical displays of estimates of a key quality process parameter. It is said that a process is working under control when the estimates fall within the range delimited by control limits, which define feasible process values that guarantee high standard quality levels. In practice, a product may work even if its quality parameters lie outside the control limits, within a range delimited by the so-called specification limits, that is, those limits that guarantee a product to be delivered and sold to customers. In such cases, the product quality will not be excellent, nevertheless adequate. In this contribution, we will give a definition of specification limits and where they are used in statistical quality control.