# [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

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"?

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

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