Modelling generic variables in a Latent class model with gmnl() - r

Question

I have the problem in fomulating a model, where at least one variable is to be estimated independently from the classes, so one and the same coefficient for all classes. How could one do this?
I am working with the R package gmnl.
install.packages("gmnl")
library(gmnl)
library(mlogit)
#browseURL("https://cran.r-project.org/web/packages/poLCA/index.html")
## Examples using the Fishing data set from the AER package
data("Electricity", package = "mlogit")
Electr <- mlogit.data(Electricity, id.var = "id", choice = "choice",
varying = 3:26, shape = "wide", sep = "")
Elec.lc <- gmnl(choice ~ pf + cl + loc + wk + tod + seas| 0 | 0 | 0 | 1,
data = Electr,
subset = 1:3000,
model = 'lc',
panel = TRUE,
Q = 2)
summary(Elec.lc)
How would you model one of the variables pf, cl, loc, wk, tod, or seas independently from the class? Thank you!

Answer 1

Thanks to Mauricio Sarrias I can present this work-around, which should solve the problem:
###################################
library("gmnl")
library("mlogit")
# Data
data("Electricity", package = "mlogit")
Electr <- mlogit.data(Electricity, id.var = "id", choice = "choice",
varying = 3:26, shape = "wide", sep = "")
# ASCs
Electr$asc2 <- as.numeric(Electr$alt == 2)
Electr$asc3 <- as.numeric(Electr$alt == 3)
Electr$asc4 <- as.numeric(Electr$alt == 4)
# We estimate a MNL for the initial values of LC-MNL
init_mnl <- gmnl(choice ~ asc2 + asc3 + asc4 + pf + cl| 0,
data = Electr)
summary(init_mnl)
# Work on initial values for LC-MNL
init <- coef(init_mnl)
Q <- 2 # Number of Classes
init.shift <- seq(-0.02, 0.02, length.out = Q)
lc.mean <- c()
for(i in 1:Q){
lc.mean <- c(lc.mean, init + init.shift[i])
}
lc.names <- c()
lc.nalpha <- c()
for (i in 1:Q){
lc.names <- c(lc.names, paste('class', i, names(init), sep = '.'))
}
names(lc.mean) <- lc.names
# Now we fix pf coefficient = 0 in the second class
lc.mean[c("class.2.pf")] <- 0
start_lc <- c(lc.mean, # Var coefficients
0) #Constant for second class
# Estimate LC with price coefficient held fixed at 0 in class 2
lc <- gmnl(choice ~ asc2 + asc3 + asc4 + pf + cl| 0 | 0 | 0 |1,
data = Electr,
model = "lc",
iterlim = 500,
start = start_lc,
fixed = c(rep(FALSE, 8), TRUE, rep(FALSE, 2)), # note that class.2.pf is fixed at 0
print.level = 3,
print.init = TRUE,
Q = 2)
summary(lc)
########################