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I ran an OLS model on a dataset with 2 categorical variables. One of them was gender. The other one had 3 different categories. I used one-hot encoding for it during pre-processing before running my model.

Variables in the image: Embarked_C and Embarked_Q. The results showed a p-value for Embarked_Q as 0.785. In this case, should I remove both Embarked_Q and C or just Q?

Regression Results

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2 Answers 2

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You should keep all of levels as they collectively describe the feature. Removing the insignificant ones will bias your coefficients and distort your interpretation (e.i. change the reference level).

Here are some stats.exchange references:

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  • $\begingroup$ Thank you. I removed the entire category and realized that it gives me the best adjusted R-squared value. I might have been lucky, but this wouldn't always be the case for every dataset. $\endgroup$ Apr 22, 2020 at 16:51
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By all means keep Embarked_C. Consider the following example: You're predicting whether or not someone's favorite color is blue. You know the color of their favorite shirt- it's either blue, yellow, or red. Color_Blue is going to be significant, the other two one hot encoded variables would not be. You'd still want to keep Color_Blue as a feature.

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  • $\begingroup$ Thanks for the answer, Brandon. I did remove all the other non-significant variables and finally, Embarked_Q. But my adjusted R-squared value decreased by 0.01. Turns out, if I remove the entire category (both C and Q), it gives me the best adjusted R-Squared value. $\endgroup$ Apr 22, 2020 at 16:48
  • $\begingroup$ I'd expect the R-squared value to drop a little bit when removing Q. The trade-off is that you're reducing the propensity to overfit when you remove less significant features. $\endgroup$ Apr 22, 2020 at 16:58

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