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Winsteps
(Rasch measurement software and publications, 2003), as its
name implies, is a psychometric program created specifically
to compute the step function (Wright & Masters, 1982)
for exams carrying partial-credit items. Despite the clarity
of the Rasch model (Bond & Fox, 2001) and the availability of
rich features, such as the item-person map, Scalogram, and
fitness indices, in the program, coding the input file for
Winteps, especially the partial-credit key, and interpretation
of Winsteps?output, could be challenging to novices. This
challenge is partly due to the Winsteps?text-based interface.
In order to augment Winsteps, two SAS macros programs were
written to aid users by formatting the input file for Winsteps
and by adding graphical presentation of the Winsteps item
parameter output.
Description
Format_PCI.sas
is a SAS Macro program (SAS Institute, 2004; Carpenter, 2004)
written to automatically format a complete control file ready
to be run by Winsteps with the proper partial-credit key
format. Users can store the key for an exam in the
format as shown in Figure 1 and the
partial-credit key will
be output as shown in Figure 2:
Figure 1.
Partial-credit key as an input string
111111111111111111111111111234 |
Figure 2.
Partial-credit key in the Winsteps format
key1=*****************************4
key2=****************************33
key3=***************************222
key4=111111111111111111111111111111 |
Nevertheless, if all items in the exam are
dichotomously-scored, the SAS program will output a binary
key. A document entitled sf.html is included with the
package to help beginners to interpret the step function
yielded from partial-credit items. Format_ICC.sas,
which is also written in SAS Macros, is a program for
generating graphical presentations, such as the Test
Information Function (TIF), Item Information Function (IIF),
Test Characteristic Curve (TCC), and Item Characteristic Curve
(ICC). This SAS program can import the question/item parameter
output and utilize its item parameter information to create
the exam-level report named report.html. This Web-based
output file presents the global overview of the exam by
illustrating TIF, TCC, and the distribution of item
parameters. Although Winsteps provides the
item-person
map with item ID and subject ID on each datum point to users,
the ID is represented by the symbol “X" or the other symbols when there are too many items and/or subjects in the
dataset. Nonetheless, the SAS graph has the feature of
revealing information at “mouse over,?namely, when the user
moves the cursor on top of one of the bars depicting an item
parameter, the item ID pops up.
In
addition, each item shown on the report page contains a
hotlink to a HTML page overlaying ICC and IIF together. If the
question is a partial-credit item, the HTML page will present
the step function in both graphical and tabulated forms. This
hyper-link feature enables test developers to visually inspect
item attributes in a more efficient fashion. In addition, the
report and graphs are Web-ready and thus the results can be
shared among colleagues through the Internet or IntraNet. A
document entitled IRT.pdf is attached to the package for
helping beginners to interpret ICC, TCC, IIF and TIF.
Both SAS
Macros programs require SAS Version 9.1.3 for Windows since
the latest features of ODS and Macros are used. Nevertheless,
even if the viewer does not have SAS in the client computer,
the SAS Graph Viewer is freely downloadable from
www.sas.com. In the second SAS program, the device driver
for outputting SAS graphs is set as "ActiveX," which is
embedded in Internet Explorer, but not in other browsers. If
the target audience does not use Internet Explorer, it is
advisable to replace "ActiveX" with "Actximg" (ActiveX Image)
or “Java." ActiveX Image renders the image in the PNG format,
which is supported by all Web browsers while Java is available
for no cost from
www.sun.com.
Availability
The
program, the documents, and the sample input/output files is
freely downloadable.
References
Bond, T.
G., & Fox, C. M. (2001). Apply the Rasch Model: Fundamental
measurement in the human sciences. Mahwah, NJ: Lawrence
Erlbaum Associates.
Carpenter,
A. (2004). Carpenter’s complete guide to the SAS Macro
Language. Cary, NC: SAS Institute.
Rasch
measurement software and publications. (2003). Winsteps
[Computer software]. [On-line] Available URL:
http://www.winsteps.com
SAS
Institute. (2004). SAS 9.1.3 [Computer Software]. Cary, NC:
The Author.
Wright, B.
D., & Masters, G. N. (1982). Rating scale analysis.
Chicago, IL: MESA Press.
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