Here we present methods to determine capabilit indices such as Cp and Cpk to be applied in "design for six sigma" (DFSS) when the quality characteristic of interest Y is unobservable but given by Y = f(X1, X2, X3,....Xk), where Xi's are random variables with means and variance respectively equal to muXi and sigma2Xi; f is a known function. The determination is based on Taylor series and we illustrate the procedure with practical examples.
Capability index of a function; error propagation; Taylor series
