Stefan H. Steiner, Grouped Data Exponentially Weighted Moving Average Control Charts, Journal of the Royal Statistical Society. Series C (Applied Statistics), Vol. 47, No. 2 (1998), pp. 203-216 ...

Although quality is often measured by the joint level of several correlated random variables, it is common to find in practice, control decisions based on individual Shewhart charts. Hotelling (1947) ...