Logistic Regression has a linear model component. If any variable can be expressed as a linear combination of others, then it will show up with 0 degress of freedom and no estimate. For instance, if you had 0-1 binary variables called Female and Male, then <Female = 1 - Male> and one of them is redundant. Usually, it is not as bovious as my example and you might need to do some two way frequencies or correlations to tease out the dependency. Doc Muhlbaier Duke
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