how to add covariates in a model matrix how to create a model matrix in r r how to create model matrix what is the model matrix for a regression how to create a model matrix in r what to do if regression has a mixed term how to create a model matrix in r how to create model matrix in r how to do the matrix model of a regression if your matrix is a covariate in r what would you use how to build a model matrix which statement is used for the matrix model? what is the matrix for a quadratic regression model matrix with variable number of covariates when to use model matrix for regression in r r create model matrix for regression how to create a model matrix in r which formula to use to create a quadratic regression model matrix how to find model matrix in model r how to create model matrix in r