When a production process involves the nesting of production units within larger production units, continuous reduction in product variability can be realized through the use of subvessel CUSUM control schemes. Control of the average levels in subvessels gives good control of vessel variability. This paper discusses the design and use of subvessel control in the chemical industry. An analysis of variance of historical process data shows that quality control should be implemented at the subvessel level to obtain the largest reduction in product variability. The example illustrates that an analysis of variance can serve as a Pareto analysis showing where product improvement projects should be carried out. A discussion of the design of a CUSUM scheme for subvessel control illustrates that such CUSUMs will often use different parameter values than CUSUMs used for vessel control. Average Run Length Tables for subvessel CUSUM design are included. These extend tables given previously.