Difference variance dispersion graphs for comparing response surface designs with applications in food technology

Variance dispersion graphs have become a popular tool in aiding the choice of a response surface design. Often differences in response from some parti cular point, such as the expected position of the optimum or standard opera ting conditions, are more important than the response itself. We describe t wo examples from food technology. In the first, an experiment was conducted to find the levels of three factors which optimized the yield of valuable products enzymatically synthesized from sugars and to discover how the yiel d changed as the levels of the factors were changed from the optimum. In th e second example, an experiment was conducted on a mixing process for pastr y dough to discover how three factors affected a number of properties of th e pastry, with a view to using these factors to control the process. We int roduce the difference variance dispersion graph (DVDG) to help in the choic e of a design in these circumstances. The DVDG for blocked designs is devel oped and the examples are used to show how the DVDG can be used in practice . In both examples a design was chosen by using the DVDG, as well as other properties, and the experiments were conducted and produced results that we re useful to the experimenters. In both cases the conclusions were drawn pa rtly by comparing responses at different points on the response surface.