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So, i was trying to improve mr LR model performing multiple linear regression on a dataset. I had a categorical variable region

Region(variable): Midwest Northeast South West

I made dummy variable for each of them and it did improved my model a bit.

Previous Model Summary enter image description here

After adding these variable(Which i made using a different variable) enter image description here

I am getting NA's in the coefficient for west, which i don't understand why. can someone explain?

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  • $\begingroup$ Which tool / framework did you get these results with? Please add it to your post title and tags if possible. $\endgroup$
    – Romain Reboulleau
    Commented Jan 13, 2020 at 5:35
  • $\begingroup$ The output is from R studio $\endgroup$
    – Ayush Ranjan
    Commented Jan 13, 2020 at 21:16

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You've given all four regions a dummy variable, so these are perfectly multicollinear, and the (unpenalized?) regression doesn't have a unique solution. R automatically drops a column in this situation and reports the NA.

https://stats.stackexchange.com/q/212903/232706
https://stackoverflow.com/q/7337761/10495893
https://stats.stackexchange.com/q/25804/232706

(You can see where R calls C and subsequently FORTRAN functions, then inserts the NAs at
https://github.com/wch/r-source/blob/0f07757ad10ca31251b28a2c332812e63c0acf38/src/library/stats/R/lm.R#L117
A nice article that helped me find that: http://madrury.github.io/jekyll/update/statistics/2016/07/20/lm-in-R.html )

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