$.$ Use visual inspection to determine whether the process mean is centered between the specification limits$.$

$2.$ Calculate the process capability index

a$)$ Use Cp if the process mean is centered between the specification limits

Cp$=$USL$-$LSL$/6$StdDev

b$)$Use Cpk if the process mean is not centered between the specification limits

Cpk$=$min$($USL$-$mean$/3$StdDev, Mean$-$LSL$/3$StdDev$)$

$3)$ Higher values of Cp and Cpk Indicate a more capable process. Experts recommend a process capability index of at least $1\mathrm{.}33$ for a two$-$sided specification. For a one$-$sided specification that consists of an upper limit only $($for example, concentration$)$ or a lower limit only $($for example, for strength$),$ the process is considered capable if the process capability index $>1\mathrm{.}25.$