Analyze Variability - Covariates
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Caution

For replicates, you account for covariates during preprocessing, so you don't include them in the model. If you have different covariate values for each replicate and include the covariate, the analysis can be inaccurate because Minitab only uses the value of the covariate that is in the same row as the standard deviation for each factor level combination.

For example, your data include four replicates. Minitab stores the standard deviation of the replicate measurements in the first row where each factor level combination appears, and enters missing data in the remaining three cells. When analyzing the model, Minitab uses the covariate value that is in the same row as the one in which the standard deviation is stored and ignores the covariate values of the other replicates.