Comparing Groups

Abstract

In Chapter 2 we learned how to test for general differences among the groups in a one-way design, and how to estimate the overall size of those differences. However, none of those techniques enabled us to tell where the differences were, that is, exactly which means were significantly different from which others. In this chapter and the next we will discuss methods of finding and testing for specific differences. In this chapter we will concentrate on testing for differences that we planned to test before obtaining the data. In Chapter 4 we will discuss ways of testing for differences that were not planned, but were suggested by the data. Both kinds of techniques are useful. In a well-planned experiment there are often specific differences in which we are interested; however, we should also be aware of unexpected differences in the data.