[R] fitted.values from zeroinfl (pscl package)

From: Sarah J Thomas <sarah.thomas_at_rice.edu>
Date: Mon, 18 Feb 2008 00:50:21 -0600


Hello all:

I have a question regarding the fitted.values returned from the zeroinfl() function. The values seem to be nearly identical to those fitted.values returned by the ordinary glm(). Why is this, shouldn't they be more "zero-inflated"?

I construct a zero-inflated series of counts, called Y, like so:

b= as.vector(c(1.5, -2))
g= as.vector(c(-3, 1))
x <- runif(100) # x is the covariate
X <- cbind(1,x)

p <- exp(X%*%g)/(1+exp(X%*%g))
m <- exp(X%*%b) # log-link for the mean process

                  # of the Poisson

Y <- rep(0, 100)

u <- runif(100)
for(i in 1:100) {

    if( u[i] < p[i] ) { Y[i] = 0 }
    else { Y[i] <- rpois(1, m[i]) }
}

# now let's compare the fitted.values from zeroinfl()
# and from glm()

z1 <- glm(Y ~ x, family=poisson)
z2 <- zeroinfl(Y ~ x|x) #poisson is the default

z1$fitted.values[1:20]
#1.3254209 0.7458029 2.0300505 1.1292954 1.4512862 #0.6513798 1.8980126
0.6558228 1.5302057
#0.6993626 2.6875736 0.7586985 2.0622238 2.1009979 #1.4254607 1.8130159
3.6603137 2.1330030
#2.9409379 3.3203350

z2$fitted.values[1:20]
#1.3587457 0.7254296 2.0730982 1.1497492 1.4902778 #0.6178648 1.9429778
0.6229478 1.5717923
#0.6726527 2.7010395 0.7400369 2.1045779 2.1424025 #1.4634459 1.8583877
3.5830697 2.1735319
#2.9354839 3.2800839

You can see that they are almost identical... and the fitted.values from zeroinfl don't seem to be zero-inflated at all! What is going on?

Ultimately I want these fitted.values for a goodness of fit type of test to see if the zeroinfl model is needed or not for a given data series. With these fitted.values as they are, I am rejecting assumption of a zero-inflated model even when the data really are zero-inflated.

many thanks,
Sarah Thomas

-- 
Sarah J. Thomas
Research Assistant, Department of Statistics
Rice University, Houston, TX

______________________________________________
R-help_at_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.
Received on Mon 18 Feb 2008 - 06:55:54 GMT

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Mon 18 Feb 2008 - 14:00:15 GMT.

Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help. Please read the posting guide before posting to the list.

list of date sections of archive