*! version 1.1 2022.01.24
help uirt_icc

Title

uirt_icc -- Postestimation command of uirt to create ICC plots and perform graphical item-fit analysis

Syntax

uirt_icc [varlist] [,options]

varlist must include only variables that were declared in the main list of items of current uirt run.
If varlist is skipped or asterisk * is used, uirt_icc will plot ICC graphs for all items declared in main list of items of current uirt run.

Options

bins(#) number of ability intervals for observed proportions; default: bins(100)

noobs suppress plotting observed proportions

pv use plausible values to compute observed proportions; default is to use numerical integration

pvbin(#) number of plausible values in each bin; default: pvbin(10000)

colors(str) list of colors to override default colors of ICC lines

tw(twoway_options) twoway graph options to override default graph layout

format(str) file format for ICC graphs (png|gph|eps); default: format(png)

prefix(str) set the prefix of file names

suffix(str) set the suffix of file names

cleargraphs suppress storing of graphs in Stata memory

Description

uirt_icc is a postestimation command of uirt that creates ICC graphs accompanied with information that aides graphical item-fit analysis.
The graphs are saved in current working directory.
Note that if ICC graphs for requested items are already saved in the working directory under default names, they will be overwritten.
If you do not want to overwrite previous ICCs, change the working directory, rename the existing files, or use prefix(str) or suffix(str) options.

Default behavior of uirt_icc is to superimpose observed proportions against the ICC curves in order to enable a graphical item-fit assessment.
The observed proportions are computed after quantile-based division of the distribution of latent variable.
Item response of a single person is included simultaneously into many intervals (bins) of theta with probability
proportional to the density of a posteriori latent trait distribution of that person in each bin.
Default method uses definite numerical integration, but after adding option pv, plausible values (PVs) will be employed to achieve this task.
Plotting observed proportions is controlled by bins() and pvbin(), it can be also turned off by noobs.
Default look of graphs can be overriden by options: colors() and tw().

Options

bins(#) sets the number of intervals the distribution of ability is split into
when calculating observed proportions of responses. Default value is bins(100).

pv changes the default method of computing observed proportions from definite numerical integration to Monte Carlo integration
with unconditioned PVs. It involves more CPU time, introduces variance due to sampling of PVs,
but takes the uncertainty in estimation of IRT model parameters into account.

pvbin(#) sets the number of plausible values used for computing observed proportions of responses
within each interval of theta. Default value is pvbin(10000).

noobs suppresses plotting observed proportions.

colors(str) is used to override the default Munsell color system used for ICC lines.
It requires a list of color names separated by spaces.
The first color in the list applies to the pseudo-guessing parameter of 3PLM - it must be declared even if there are no 3PLM items in the model.

tw(twoway_options) it is used to add user-defined twoway graph options to override the default graph layout,
like: xtitle() or scheme() etc.

format(str) specifies the file format in which the ICC graphs are saved (png|gph|eps).
Default value is format(png).
This option influences also the graphs created after asking for DIF analysis.

prefix(str) is used to define a string that is added at the beginning of the names of saved files.
Default value is prefix(ICC).

suffix(str) adds a user-defined string at the end of the names of saved files.
Default behavior is not to add any suffix.

cleargraphs is used to suppress the dafault behaviour of storing all ICC graphs in Stata memory.
After specifying this, all graphs are still saved in the current working directory, but only the last graph is active in Stata.

Examples

Setup

. webuse masc1

Fit an IRT model using all items in the dataset with default settings of uirt

. uirt q*

create ICC graphs for all items used in the above command - these will be stored as PNG files in current working direcrory

. uirt_icc

create ICC graph only for item q1, change graph title, the name of x-axis, and the color of the ICC line, add suffix "xx" at the end of file name

. uirt_icc q1,tw(title(ICC graph for item 1) xtitle(My preferred name for {&theta} variable)) color(any_for_c green) suff(xx)

Stored results

uirt_icc does not store anything in r():

Author

Bartosz Kondratek
everythingthatcounts@gmail.com