Lecture 14 - 15 MIXED NEW

Lecture 14 15 - Use of Proc Mixed to Analyze Experimental Data Animal Science 500 Lecture No October 2010 IOWA STATE UNIVERSITY Department of

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I OWA S TATE U NIVERSITY Department of Animal Science Use of Proc Mixed to Analyze Experimental Data Animal Science 500 Lecture No. October , 2010
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I OWA S TATE U NIVERSITY Department of Animal Science GLM and MIXED in SAS The SAS procedures GLM and MIXED can be used to fit linear models. Commonly used to analyze data from a wide range of experiments Proc GLM was designed to fit fixed effect models Later amended to fit some random effect models by including RANDOM statement with TEST option. The REPEATED statement in PROC GLM allows to estimate and test repeated measures models with an arbitrary correlation structure for repeated observations.
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I OWA S TATE U NIVERSITY Department of Animal Science GLM and MIXED in SAS The PROC MIXED was specifically designed to fit mixed effect models. Including fixed effects, random effects, repeated effects It can model data with heterogeneous variances and autocorrelated observations. The MIXED gives a user more flexibility in specifying the correlation structures, particularly useful in repeated measures and random effect models.
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I OWA S TATE U NIVERSITY Department of Animal Science GLM and MIXED in SAS SAS has made many advancements to different procedures over the years. PROC MIXED is not an extension of GLM. Based on different statistical principles; GLM and MIXED use different estimation methods GLM uses the ordinary least squares (OLS) estimation Parameter estimates are such values of the parameters of the model that minimize the squared difference between observed and predicted values of
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I OWA S TATE U NIVERSITY Department of Animal Science GLM and MIXED in SAS Using PROC MIXED you do not get the analysis of variance table that you may be used to evaluating. It uses estimation methods based on different principles. PROC MIXED has three options for the method of estimation. They are: ML (Maximum Likelihood), REML (Restricted or Residual maximum likelihood, which is the default method) and MIVQUE0 (Minimum Variance Quadratic
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I OWA S TATE U NIVERSITY Department of Animal Science Defining fixed or random factor Random Fixed Levels Selected at random from a conceptually infinite collection of possibilities Finite number of possibilities Another experiment Would use different levels from the same population Would use the same levels of the factor Goals Estimate variance components Estimate means Inference For all levels of the factor (i.e. for population from which levels are selected) Only for levels actually used in the experiment From D. A. Dickey, 2008: SAS Global Forum
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I OWA S TATE U NIVERSITY Department of Animal Science Characteristics of Mixed Solutions Best Linear Unbiased Estimates = BLUE Best Linear Unbiased Predictions = BLUP
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I OWA S TATE U NIVERSITY Department of Animal Science Comparison with GLM GLM has a random statement however it models all effects as if they were fixed
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I OWA S TATE U NIVERSITY Department of Animal Science
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Lecture 14 15 - Use of Proc Mixed to Analyze Experimental Data Animal Science 500 Lecture No October 2010 IOWA STATE UNIVERSITY Department of

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