Helen

Your program makes no sense. 

Try the following script

I <- 10
J <- 5
response <- matrix(0, 10, 5)

#function for probability of response
#revised but equivalent version of Helen's formula

pij <- function(a,b,theta) {1/(1+exp(-a*(theta-b)))} 

a <- rnorm(J, 0.8, 0.04)
a
b <- rnorm(J, 0, 1)
b
theta <- rnorm(I, 0,1)
theta

for( i in 1:I ) {
 for( j in 1:J ) {
  response[i,j]<-ifelse(pij(a=a[j], b=b[j], theta[i]) < runif(1) , 0 ,1)
 }
}

response





Helena <helenagucheng_7@hotmail.com> 
Sent by: r-help-bounces@r-project.org
03/10/2010 11:03 PM

To
r-help@stat.math.ethz.ch
cc

Subject
[R] about IRT simulation






hello R:
we have a two-parameter IRT simulation code. The goal is to generate a 
response matrix.But the "for" part doesn't run. we don't know what is 
wrong 
with it.

Thanks so much~~~

I <- 10
J <- 5
response <- matrix(0, 10, 5)
pij <- function(a,b,theta)
{
a <- rnorm(J, 0.8, 0.04)
a
b <- rnorm(J, 0, 1)
b
theta <- rnorm(I, 0,1)
theta
for( i in 1:I ) {
                 for( j in 1:J ) {
ptemp <- runif(1)
                                 pij <- 
exp(a[j]*(theta[i]-b[j]))/(1+exp(a[j]*(theta[i]-b[j])))
                 response[i,j]<-ifelse(pij(b=b[j], a=a[j], theta[i]) < 
ptemp , 0 ,1)

}
}
}
response 



helena

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