Increasing the number of levels of an independent variable. Experiment may with 3- more levels for several reasons . Design with only two levels of independent variable cannot provide much info. about exact form of relationship between independent and dependent variables . Relationship is monotonic positive relationship, rather than strictly linear relationship . Two levels only have a solid line . Variables are sometimes related in curvilinear/ non-monotonic fashions . The direction of relationship changes (e. g. inverted-u) . Experiment with only 2 levels of independent variable cannot detect curvilinear relationship . At least need 3 levels in experimental design . Increasing the number of independent variables: factorial design. Two/three independent variables are operating simultaneously . Closer to real-world conditions, which independent variable do not exist by themselves . Factorial design: designs with more than one independent variable . All levels of each independent variable are combined with all levels of the other independent variables .