There have been some publications dedicated to
the design of control charts with linguistic or fuzzy
data. In the case of monitoring unique fuzzy quality
characteristics, Wang and Raz (1990) as well as Raz
and Wang (1990) were the first authors who proposed a
control chart for linguistic data (fuzzy data). They pointed
out that linguistic data can provide more information
than the binary classification used in control charts by
attributes. They proposed representative values control
charts with both the probability rule and the membership
function rule, for which the linguistic data (fuzzy data)
are transformed into scalars referred to as representative
values of the fuzzy data, and these representative values
are plotted on an ordinary control chart. In their paper
four kinds of transformation formula have been proposed,
i.e., the fuzzy mode, fuzzy midrange, fuzzy median and
fuzzy average.