A fractional factorial experiment is generated from a full factorial experiment by choosing an "alias structure". The alias structure determines which effects are confounded with each other. For example, the five factor 25 − 2 can be generated by using a full three factor factorial experiment involving three factors (say "A"," B", and "C") and then choosing to confound the two remaining factors "D" and "E" with interactions generated by "D" = "A"*"B" and "E" = "A"*"C". These two expressions are called the "generators" of the design. So for example, when the experiment is run and the experimenter estimates the effects for factor "D", what is really being estimated is a combination of the main effect of "D" and the two-factor interaction involving "A" and "B".