There are different types of classifications of control charts, such as parametric
versus non-parametric, variable versus attributes, univariate versus multivariate
and bayesian versus classical. The parametric charting structures rely on the
assumption that the parent distribution of the quality characteristic of interest is
known (more specifically normally distributed in many cases). This assumption is
not very realistic as, in practice, one may not know the distributional form of the
quality characteristic of interest.