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variance_inflation_factor for constants

variance_inflation_factor for constants

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I am using the function variance_inflation_factor from statsmodels.stats.outliers_influence for calculating VIF. My questions are,
 

  1. Should we consider the constant (added with add_constant, as statsmodels doesn’t add constant by default) while calculating VIF?
  2. What does a very high value of a VIF of constant suggest . My constant (or intercept) has a VIF of 70-100.
  3. Should we drop the constant (or intercept)  if the VIF is high but it is significant (i.e. p-value is low) ?

Note : All my variables are min maxed scaled except the categorical variables and the dependent variable.

1 Answer

365 Team
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Hi Abhishek,
Thanks for reaching out.

  1. Conceptually it doesn’t matter if we add or remove the constant from the VIF calculation. Overall it doesn’t make a difference for the model, because the constant is not a predictor.
  2. A constant has irrational VIF values as it is not supposed to be calculated/factored.
  3. No, you should not drop the constant. You should simply disregard the VIF result you get for the constant.

Best,
The 365 Team

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