When the parameters μ and σ of the underlying process are unknown, some
assumptions are made to construct an Xbar control chart whose performance is close to
one developed with known parameters. As mentioned previously, the development
usually is done in two phases designated Phase I and Phase II. Control limits are
calculated using parameter estimates from an in-control Phase I historical sample. .In
Phase II statistics based on new samples are compared with these limits monitoring to
detect out-of-control situations