The function factor.scores does not "inherit" anything. It is a generic
function that provieds methods for a number of classes, including those you
mention. (The terminology is important if you are to understand what is going
on here):
> library(ltm)
Loading required package: MASS
Loading required package: msm
Loading required package: mvtnorm
Loading required package: polycor
Loading required package: sfsmisc
This is package 'ltm' version '0.9-5'
> find("factor.scores")
[1] "package:ltm"
> factor.scores
function (object, ...)
{
UseMethod("factor.scores")
}
<environment: namespace:ltm>
> methods("factor.scores")
[1] factor.scores.gpcm factor.scores.grm factor.scores.ltm
factor.scores.rasch
[5] factor.scores.tpm
>
So what you have to do, if you want factor.scores() to work on objects that you
create is two things:
1. Give your objects an S3 class attribute, say "bifactor", and
2. Write your own S3 method function
factor.scores.bifactor <- function(object, z, y, x, ...) { .... }
to perform the task. The name of the function has to be
"factor.scores.bifactor" and the name of the first formal argument has to be
"object" and you must have a ... in the argument list, but from that point on
you have a pretty free hand.
What you probably want to do is use some of the existing method functions to do
the hard work once you have munged you object into a form that makes that
possible.
Bill Venables.
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf Of James Lewis
Sent: Thursday, 10 February 2011 12:09 PM
To: r-help@R-project.org
Subject: [R] factor.scores
The function factor.scores is used with package ltm and others to estimate IRT
type scores for various models. It inherits objects of class grm, gpcm and a
few others. What I would like to do is to use the factor.scores function, but
feed it my own item parameters (from a bifactor model where the 2PL parameters
are adjusted for the bifactor structure). Does anybody have an idea of how this
might be done? I can, of course, create a list, matrix or other object
containing the item parameters, but factor.scores only inherits particular
object classes. Any thoughts would be great. Thanks,
Jimmy
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