Hi:

Here's a simple example of its use using the data from Montgomery, Myers and
Vining (2002), problem 4.7:

ex4.7 <- structure(list(pressure = c(2500L, 2700L, 2900L, 3100L, 3300L,
3500L, 3700L, 3900L, 4100L, 4300L), nfast = c(50L, 70L, 100L,
60L, 40L, 85L, 90L, 50L, 80L, 65L), nfailures = c(10L, 17L, 30L,
21L, 18L, 43L, 54L, 33L, 60L, 51L)), .Names = c("pressure", "nfast",
"nfailures"), row.names = c("1", "2", "3", "4", "5", "6", "7",
"8", "9", "10"), class = "data.frame")

# nfast is the sample size per covariate class, nfailures and pressure
# the response and covariate, respectively.

# Notice that the weights variable is the denominator of the response
m <- glm(nfailures/nfast ~ pressure, data = ex4.7, weights = nfast)

ex4.7a <- cbind(ex4.7, pred = predict(m, type = 'response'))

# Check that the observed proportions and the predicted probabilities are
reasonably in sync

library(ggplot2)
ggplot(ex4.7a, aes(x = pressure, y = nfailures/nfast)) +
    geom_point(size = 2.5) + geom_line(aes(y = pred), size = 1, color =
'red')

# or in base graphics,
plot(nfailures/nfast ~ pressure, data = ex4.7a, pch = 16)
with(ex4.7a, lines(pressure, pred, col = 'red', lwd = 1.2))

HTH,
Dennis

On Sat, Dec 11, 2010 at 8:59 AM, Enrique Garcia <eckomorph@yahoo.com> wrote:

>
> Hello R folks,
>
> I have three questions. I am trying to run a logistic regression (binomial
> family) where the response variable is a proportion.  According to R
> Documentation in "a binomial GLM prior weights are used to give the number
> of trials when the response is the proportion of successes."  However when
> I
> run my code I get the following error message:
>
> Error in model.frame.default(formula = PER_ELA ~ A_EX + COMM + ENG + S_R +
> :
>  variable lengths differ (found for '(weights)')
>
> I'm not sure what I am doing wrong.  My response variable is Y/M, which is
> the proportion of 1's (successes) among M binary responses.  My prior
> weight
> is a variable indicating the number of trials for each observation.
>
>
> This is an abbreviated version of the code that I ran:
>
> glm1<-glm(PER_ELA~A_EX .... PER_LEA,
> family=binomial(link="logit"),data=data2,weights="REG")
>
>
> Question 1 and 2:
> Does the number of trials for each observation in my dataset have to be the
> same? What am I doing wrong here?
>
>
> Question 3:
> Is it OK for me to use percentages as predictor variables in a logistic
> regression?
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/Specifying-Prior-Weights-in-a-GLM-tp3083480p3083480.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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