For example, if inference is to be made about yields of wheat cultivars when grown as a field crop, the experimental unit should be a field of wheat. The individual plants within the field do not behave indepen dently of each other as they compete with each other for light and nutri ents. If inference is to be made about growth of animals when maintained as a group then the experimental unit is the group of animals. Just as individual plants within a field do not behave indepen dently, individual animals within the group do not behave indepen dently of other members of the group. They interact socially with each other as they too compete for space and nutrients. However, it is impor tant for the response in the experimental units, which are the field of plants or the group of animals, not to be influenced by the responses to treatments applied to other fields of plants or group of animals in the experiment.