Differences between EPDIRM and ICL

The computer program IRT Command Language (ICL) supercedes the Estimation Program for Dichotomous IRT Models (EPDIRM). ICL is really a new version of EPDIRM, with the name changed due to the fact that ICL can estimate item parameters for polytomous as well as dichotomous IRT models. Many commands in the last released version of EPDIRM (version 0.011118) remain unchanged in ICL, although there are cases where the name or arguments of a command changed. This page documents command differences in EPDIRM and ICL with the purpose of assisting EPDIRM users in migrating to ICL. The page does not describe every difference in commands between EPDIRM and ICL, but describes the major differences that should allow most EPDIRM command files to be modifed for use with ICL.

Commands with Changed Names

There are several commands whose names changed in ICL, but which function identically in EPDIRM and ICL. The following table gives EPDIRM commands whose name changed and the names of the corresponding command in ICL.

EPDIRM	ICL
epdirm_start	allocate_items_dist
epdirm_end	release_items_dist
new_epdirm	new_items_dist
num_theta	num_latent_dist_points
delete_epdirm	delete_items_dist
item_starting_values	item_3PL_starting_values
set_default_model	set_default_model_dichotomous, set_default_model_polytomous
starting_values	starting_values_dichotomous
write_item_param	write_item_param_channel
write_latent_dists	write_latent_dist_channel

For the write_latent_dists and write_item_param commands some arguments of the commands changed as described in the next section.

Commands with Changed Arguments

There were some cases in which the arguments of a command changed, even though the command name is the same in EPDIRM and ICL. The first argument of the commands item_get_param, item_get_prior_type, and item_get_prior_param identifies the item parameter on which the command operates. In EPDIRM this argument was a character identifying a parameter name. In ICL this argument is a index identifying the position of the parameter in the list of parameters for the item.

Positional arguments in the EPDIRM commands write_item_param (named write_item_param_channel in ICL), write_latent_dist (named write_latent_dists_channel in ICL), read_item_param, and read_latent_dist are optional arguments in ICL.

The positional argument -num_theta and -theta_range of the EPDIRM epdirm_start command have been changed to -num_latent_dist_points and -latent_dist_range, respectively, in the ICL allocate_items_dist command.

New Commands in ICL

The following are some of the commands that were added to ICL.

examinee_group - Returns the group an examinee belongs to.
examinee_response_str - Returns the responses for an examinee in a string, were each character represents the response to one item.
item_get_all_params - Returns all parameters for the item, including estimated and fixed parameters. An example of a fixed parameter is the c-parameter in the two parameter logistic model.
item_set_all_params - Set values of all parameters for an item, including estimated and fixed parameters.
write_item_param_file - Same as write_item_param, but takes a file name as the first argument rather than an I/O channel returned by the Tcl open command.
write_latent_dist_file - Same as write_latent_dist, but takes a file name as the first argument rather than an I/O channel returned by the Tcl open command.
read_item_param - Same as read_item_param_file (which was named read_item_param in EPDIRM) but takes an I/O channel returned by the Tcl open command as the first argument rather than a file name.
read_latent_dist - Same as read_latent_dist_file (which was named read_item_param in EPDIRM), but takes an I/O channel returned by the Tcl open command as the first argument rather than a file name.
test_characteristic_curve - Returns the value of the test characteristic curve at specific theta values.
item_prob_resp - Returns the probability an examinee with a given value of the latent variable would respond in a particular response category.
dist_transform - Transform the scale of the points for the latent variable distribution.

Bugs in EPDIRM Fixed in ICL

Some bugs in EPDIRM version 0.011118 that are fixed in ICL are:

Fixed a bug in read_examinee_missing command that prevented the command from working when the optional fifth and sixth arguments were used.
Fixed a bug in the item_set_model command which caused the command to not work and possibly crash the program.
Fixed a bug in the simulate_responses command that produced an error when simulate_responses was called.
Fixed a bug in which the examinee counts in each group were not updated in bootstrap_sample.

Software by Brad Hanson

Brad Hanson's Home Page

URL of this page: http://www.b-a-h.com/software/irt/icl/epdirm_differences.html

Last updated: March 1, 2002