Random Effects

In the previous chapters, we assumed that scores within a group were randomly selected. However, the groups from which scores were to be taken were assumed to have been the deliberate choice of the experimenter. For the data in Table 4.1, for example, the two drugs were assumed to have been chosen because we were particularly interested in those two drugs. They were not assumed to have been selected randomly from a large population of potential drugs. In some cases, however, the groups or “treatments” themselves may have been selected randomly from a large number of potential treatments. In this chapter we will consider methods for analyzing such data.