R Logistic Regression Missing Coefficients

问题

I am trying to asses the odds of people staying in a program given their backgrounds following these instructions. One of the variables I am looking at is age, which I split into five groups. I have run a test using the formula:

mylogit15 <- glm(Stay_in_Progams ~ Age.Group + Prior_Experience,
                 data = mydata, family = "binomial")

The results of the test are clear enough, except I am missing the first and third age groups. This is what they look like:

Coefficients:

             Estimate        Std. Error    z-value      Pr(>|z|)
(Intercept)   -.298             1.173      -1.98          .047
Age Group2    1.201             1.243       0.966         .333
Age Group4    2.735             1.486       1.840         .065
Age Group5    1.636             1.673       0.965         .334
Prior_Exp     3.546             1.234       2.735         .006

Thank you for taking the time to read this and help me out!

来源：https://stackoverflow.com/questions/31930261/r-logistic-regression-missing-coefficients