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1 Introduction 21.1 What ASReml can doASReml (pronounced A S Rem el) is used to fit linear mixed models to quite largedata sets with complex variance models. It extends the range of variance modelsavailable for the analysis of experimental data. ASReml has application in theanalysis of• (un)balanced longitudinal data,• repeated measures data (multivariate analysis of variance and spline type models),• (un)balanced designed experiments,• multi-environment trials and meta analysis,• univariate and multivariate animal breeding and genetics data (involving arelationship matrix for correlated effects),• regular or irregular spatial data.The engine of ASReml forms the basis of the REML procedure in GENSTAT andthe asreml class of S language functions (Butler et al. 2007) available for S-Plus(ASReml-S) and R (ASReml-R) 1 . Both of these have good data manipulation andgraphical facilities. and will be adequate for many analyses, some large problemswill need to use ASReml. The ASReml user interface is terse. Most effort hasbeen directed towards efficiency of the engine. It normally operates in a batchmode.Problem size depends on the sparsity of the mixed model equations and the sizeof your computer. However, models with 500,000 effects have been fitted successfully.The computational efficiency of ASReml arises from using the AverageInformation REML procedure (giving quadratic convergence) and sparse matrixoperations. ASReml has been operational since March 1996 and is updated periodically.1.2 InstallationInstallation instructions are distributed with the program. If you require helpwith installation, please email support@asreml.co.uk.1 These are available from VSN (http://www.vsni.co.uk).