R help - Sep 2007 - How to explain the meaning of mu in the variance function of GLMs?

Dear R friends,
  When fitting GLMs in R, we may need to specify the variance function to do
our analysis. I had thought it's the mean value, but it seems not. Could
anybody expain the correct meaning of *mu* in the variance function of
GLMs?  The following content is from the R-hlep.

  variance for all families other than quasi, the variance function is
determined by the family. The quasi family will accept the literal character
string (or unquoted as a name/expression) specifications "constant",
"mu(1-mu)", "mu", "mu^2" and "mu^3", a
length-one character vector taking
one of those values, or a list containing components varfun, validmu,
dev.resids, initialize and name.
 Thanks very much.
-- 
With Kind Regards,

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Zhi Jie,Zhang ,PHD
Tel:86-21-54237149
Dept. of Epidemiology,School of Public Health,Fudan University
Address:No. 138 Yi Xue Yuan Road,Shanghai,China
Postcode:200032
Email:epistat@gmail.com
Website: www.statABC.com
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