Using the Cook and Campbell (1979) notation, you can add more groups or more observations to provide more opportunities for comparison. It is the capacity to make inferences about variation within and between groups that is central to explanatory, or hypothesis testing, research. In fact, there are really only four questions asked about the dependent variable. Did it go up? Did it go down? Did it stay the same? Was any change caused by the independent variable? It is the last of these questions that requires all of the opportunities for comparison.