Published articles are of immediate and practical value to applied researchers and statistical consultants in these fields because only papers addressing applied statistical problems are considered. The journal contributes to the development and use of statistical methods in the agricultural sciences, the biological sciences (including biotechnology), and the environmental sciences (including those dealing with natural resources). Published quarterly since 1996, the Journal of Agricultural, Biological, and Environmental Statistics (JABES) is a joint publication of the American Statistical Association and the International Biometric Society. Cubic smoothing splines are used to describe differences in the trend over time and unstructured covariance matrices between times are found to be necessary. We also describe the mixed model analysis of data from a three-phase experiment to investigate the effect of time of refinement on Eucalyptus pulp from four different sources. The method is applied to formulate mixed models for a wide range of examples. This is done by dividing the factors into sets, called tiers, based on the randomization and determining the crossing and nesting relationships between factors. At the same time we retain the principle that the model used should include, at least, all the terms that are justified by the randomization.
Our approach extends an earlier method by Brien and Bailey to explicitly identify terms for which autocorrelation and smooth trend arising from longitudinal observations need to be incorporated in the model. This article describes a method for deriving the terms in a mixed model. Mixed models have become important in analyzing the results of experiments, particularly those that require more complicated models (e.g., those that involve longitudinal data).
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