The 3-Sigma Lower Control Limit (LCL) for an R Chart $\bar { r } - 3 c _ { n } \bar { r } / b _ { n }$

Short Answer

The 3-sigma lower control limit (LCL) for an R chart is given by LCL = $\bar { r } - 3 c _ { n } \bar { r } / b _ { n }$ where the values of $b _ { n } \text { and } c _ { n }$ for n=3,…..,8 are tabulated in your text. This expression for LCL will be negative if $n \leq$ __________, in which case it is customary to use LCL = __________.