Using NA in factors used in the design matrix for glm()
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Assume I want a categorical variable (called c in the example below) to be connected with the first half of the elements in my response variable and not influence the second half of the response variable. At first I thought I could do it by using NA in the positions I don't want it to be connected, as in the example below:

    y = rpois(10, lambda = 1)
    x = runif(10)
    c = c(c("a", "b", "b", "a", "a"), rep(NA, 5))

    data = data.frame(y,x,c)

    r = glm(formula = y ~ -1 + c + x, family = "poisson", data = data)

But this doesn't work since glm remove the lines that contain "NA". Any help?