R help - Nov 2007 - rank-deficient model matrix

Dear:
      I want to construct a gee model in R. When I ran the program, there was a
warning in the output.
The warning is "Beginning Cgee S-function, @(#) geeformula.q 4.13 98/01/27
                       gee(cbind(hyper, nohyper) ~ I(Ethnic) + I(Gender) +
I(drink) +  :
                       rank-deficient model matrix"
What is the rank-deficient model matrix? What should I do to solve the problem?
Thanks all.
                                              Lian


     
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