In this section, we will discuss how treatments are constructed from several factors rather than just being (t) levels of a single factor. These types of experiments are involved with examining the effect of two or more explanatory variables on a response variable (y). For example, suppose a company has developed a new adhesive for use in the home and wants to examine the effects of temperature and humidity on the bonding strength of the adhesive. Several treatment design questions arise in any study. First, we must consider what factors (explanatory variables) are of greatest interest. Second, the number of levels and the actual settings of these levels for each of the factors must be determined for each factor. Third, having separately selected the levels for each factor, we must choose the factor–level combinations (treatments) that will be applied to the experimental units.