In a previous article, we discussed how to select experiments for factorial designs.1 Factorial designs are best used when studying categorical and continuous variables. Categorical variables include non-numeric variables, like lots of raw materials, equipment, methods and personnel. Categorical factorial designs are even used in the social sciences to differentiate between different modes of education or psychiatric treatment. A factorial design can tell if units within a category are statistically different. Continuous variables are numeric variables, like time, temperature, speed and pressure. Factorial designs on continuous variables can be used to draw response surfaces and quantify experimental variation. They are used to optimize manufacturing procedures and stabilize test methods.