1. If the regression model includes several categorical variables,is it rational to calculate VIF (Variance Inflation Factor) without converting them to dummy variables?

2. Is there a way to calculate VIF from variance-covariance matrix of regression coefficient without calculating statistics regarding predictors ? (I need this because  Matlab regression gives cov(b) and handles categorical variables internally,but does not give the predictors variance. Please suppose variables are not mean centered) 

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