Exploration & descriptionContents Fixed and random effects Before incorporating ram and ewe random effects into the statistical model it is worth discussing first the meaning of mixed models. Mixed model methodology takes its name from the understanding that the elements of the model underlying a statistical analysis can be a mixture of what are called fixed and random effects. The approach has become important in the analysis of data that have a hierarchical structure, since the different layers in the structure can be modelled using random effects. A fundamental step in using mixed models for hierarchical data is to recognise the structure, namely the different layers in the data. In order to help with this we shall use what we describe as a ´mixed model tree´ to develop the different layers pictorially. This is also illustrated in the statistical guide by Allan and Rowlands (2001) which uses the data from this case study for one of its examples. This guide is no.19 of the Good Practice Guides. It also includes examples from Case Study 6 and from the paper by Methu et al (2001). |
