I am running an ordinal logistic regression model in R. My dependent variable is a categorical variable with 4 levels. I have six independent variables. When I run the ordinal logit, one of the six independent variables gets split into two with each having its own coefficient.

I was wondering why this could be happening.

Any assistance or answers would be greatly appreciated.

Thanks, Shawn

  • $\begingroup$ Please share output so we can more easily interpret what you are looking at. $\endgroup$ – robin.datadrivers Jun 2 '17 at 19:02

one of the six independent variables gets split into two

If you check this independent variable is that a factor and has three levels? If yes, it is called categorical variable coding. Details can be found here.

Here is a simple demo. Note, the cyl is number of cylinders in a car, and it is a factor has 3 levels.


glm(formula = am ~ wt + factor(cyl), family = binomial(), data = mtcars)

Deviance Residuals: 
     Min        1Q    Median        3Q       Max  
-1.83559  -0.29948  -0.04014   0.19966   1.98747  

             Estimate Std. Error z value Pr(>|z|)   
(Intercept)    20.853      8.032   2.596  0.00942 **
wt             -7.859      3.055  -2.573  0.01009 * 
factor(cyl)6    3.105      2.425   1.280  0.20042   
factor(cyl)8    5.379      3.201   1.681  0.09281 . 
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Dispersion parameter for binomial family taken to be 1)

    Null deviance: 43.230  on 31  degrees of freedom
Residual deviance: 14.661  on 28  degrees of freedom
AIC: 22.661

Number of Fisher Scoring iterations: 7

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