Therefore, traditional control charts, based on precise
data to monitor processes, have to be expanded in
order to possibly carry out process monitoring tasks in
imprecise or fuzzy environments The extended attribute
control charts such as χ2-control charts for multi-label
categories or control charts for grouped data may be
suitable for this purpose, but only if the vague data could
be expressed approximately in distinct categorical form
and the distributions of the underlying process variables
are known (Woodall and Tucker, 1997).