R summary(glm) - Coefficients: (1 not defined because of singularities)

Question

I use the glm fonction for different logisitc regression with binomial data. When I run the summary(glm) I got the following output :

My problem is that the last line is noted "NA" for all my glm. For exemple here FRUIT is "NA" but if I change the command and put BRANCH at the end BRANCH gonna be noted "NA". So the problem is not the data, it's just the last line and I don't understand why...

If you need more informations to uderstand, let me know ! Thank you so much for your help.

glm(formula = COLLETO ~ LEAF + BRANCH + FRUIT, family = binomial(link = "logit"), 
data = or)

Deviance Residuals: 
Min       1Q   Median       3Q      Max  
-0.8363  -0.6203  -0.6203  -0.2520   2.6328  

Coefficients: (1 not defined because of singularities)
        Estimate Std. Error z value Pr(>|z|)    
(Intercept)  -0.8708     0.2807  -3.102  0.00192 ** 
LEAF         -0.6798     0.3169  -2.145  0.03197 *  
BRANCH       -2.5632     0.5341  -4.799 1.59e-06 ***
FRUIT             NA         NA      NA       NA    
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Answer 1

The issue here is that the last coefficient in your model is being dropped because of collinearity. Essentially, the model is recognizing that two or more of your predictors are identical, or perfectly predicted by the combination of the other two predictors. This means that you cannot include all three terms in your model.

This question will be useful for you: How to deal with an error such as "Coefficients: 14 not defined because of singularities" in R?