[datatable-help] Predictions and Poisson

[sarah.harding]

I have been asked to do the following task:
/'Using the data-frame in DoctorVisits.Rda, perform a Poisson regression
analysis of how
number of visits to the doctor in the two week period varies as a function
of age group (i.e.
<30, 30-50, >50), sex and illness. Holding sex and number
of illnesses constant at their mean values, what are the predicted rates of
visits to a doctor
over a two week period for different age groups.' /

I have ran my Poisson regression... 
M.dr <- glm(visits ~ age.category + gender + illness,
            +             data=DoctorVisits,
            +             family=poisson)

But now I am stuck on how to input gender as a constant mean in the
prediction function. So far I have...
 predictor.values <- with(DoctorVisits,
data.frame(age.category=c('<30', '30-50','>50'), illness=mean(illness),
gender= *??????*)

I do not know what to put in for the gender variable as I am dealing with
factors. Will be grateful for the help. Thanks. 




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