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equation help

3 messages · Brendan Morse, Doran, Harold, David Winsemius

Brendan Morse
# 
Hi everyone, I am having a bit of trouble correctly structuring an  
equation in R.


Here is the equation



Here is what I thought

for(i in 1:numItem)for(x in 1:numCat)

           Ptheta[,i,x]<-(exp(-1.702*a[i]*(theta-b[i,x+1]))

My problem is that I am not sure how to get it to read the equation as  
having two indexing points (i and x). Right now it is trying to read  
it as a matrix, but each value (i and x) are only vectors. Does anyone  
have any suggestions on how to get R to recognize this? As an example,  
numItem may = 20 and numCat may = 3 and I am trying to evaluate this  
over all values of numCat for each value of numItem.

Thanks,
Brendan
Doran, Harold
# 
Brendan

It looks like you're working with the 2PL, what are you trying to
estimate exactly? There are a lot of built in psychometric functions
that you might consider using.
David Winsemius
#
On Apr 13, 2009, at 10:54 AM, Brendan Morse wrote:

            

      
      
      
    

        
It's really a series of assignments.

            

      
      
      
    

        
I am guessing that what you really want is:

for(i in 1:numItem){
          for(x in 1:numCat){
                    Ptheta[i,x]<-(exp(-1.702*a[i]*(theta-b[i,x+1]))
                    } }  #obviously untested

That should populate the i, x entries of a matrix with your  
calculations. The extra comma in your attempt was telling R that you  
wanted a three dimensional array. The other possibility as a  
construction would be to use expand.grid

            

      
      
      
    

        
Each value is an integer by the time it gets to the innards of the loop.