D2. IRT Models questions

BILOG-MG questions

Q1. What about running BILOG-MG for a single group?

A1: The IRT tutorial web site currently provides sample syntax that can be used on BILOG for DOS and BILOG for Windows (e.g., version 3.1) only. As BILOG-MG uses a slightly different syntax from the previous two versions of BILOG, you may use the examples provided here and modify the syntax by referring to the examples provided within the Help file in BILOG-MG. A sample BILOG-MG syntax is provided below.

Q2. What about running BILOG-MG for multiple groups (e.g., DIF)?

A2: When using BILOG-MG to perform multiple groups DIF analysis, it seems that the dataset has to be formatted differently. For instance, group identification code should be inserted between the Subject ID and item scores. We may have sample BILOG-MG syntax codes updated here soon.

Q3. I got an error message that "the key for item XXX is not the same as its key in an earlier form", but I have checked the key file a million of times and the key answer is similar in both forms!

A3: Please check not only your key file, but also your data file.

Q4. The *.COV files I got from BILOG-MG is different from the files you provided in the tutorial. What do those numbers mean?

A4: The format of BILOG-MG's estimated covariance file is formatted slightly different from BILOG for DOS/Windows's estimated covariance file. That is, the order for each record remains the same, but the location for each record is formatted differently. Please refer to BILOG-MG manual pages 248 and 249 for details.

Lines 1 & 2 contains the title records of the BILOG-MG run that created the covariance file. Afterwards, there are two lines for each item. The first record contains

(1-8) item name,

(9-16) subtest name,

(17-21) group indicator,

field 1: (22-33) slope estimate (a),

field 2: (34-45) threshold estimate (b),

field 3: (46-57) lower asymptote estimate (c),

field 4: (58-69) estimation of error variance for slope (SEa2), and

field 5: (70-81) estimation of error covariance for slope and threshold (Cova,b).

The second record contains

(1-17) blank filler,

field 6: (18-29) estimation of error variance for threshold (SEb2),

field 7: (30-41) estimation of error covariance for slope and asymptote (Cova,c),

field 8: (42-53) estimation of the error covariance for threshold and asymptote (Covb,c), and

field 9: (54-65) the estimate of the error variance for lower asymptote (SEc2).

Q5. How can I convert BILOG-MG generated *.PAR file for further analysis (e.g., ITERLINK) or other purposes (e.g., free-field format)?

A5: You may use a new program called PAR3PLMG. See Section F. DIF/DTF questions below.

Q6. How can I convert BILOG-MG generated *.COV file for further analysis (e.g., ITERLINK) or other purposes (e.g., BILOG for DOS/Windows format)?

A6: You may use a new program called COVMG.

Q7. Can you provide a sample BILOG-MG syntax?

A7: BILOG-MG syntax may look like this:

> AGREEABLENESS CALIBRATION FOR IRT TUTORIAL

> COMMENT

> GLOBAL DFNAME='AGR2_CAL.DAT', NPARM=3, SAVE;

> SAVE SCORE='AGR2_CAL.SCO', PARM='AGR2_CAL.PAR', COV='AGR2_CAL.COV';

> LENGTH NITEMS=(10);

> INPUT SAMPLE=99999, NIDCHAR=4, OFNAME='OMIT.KEY';

> ITEMS INAMES=(AGR1(1)AGR10);

> TEST1 INUMBER=(1(1)10);

(4A1,10A1)

> CALIB NQPT=40, CYCLES=100, NEWTON=30, CRIT=.001, PLOT=0;

> SCORE MET=2, IDIST=0, RSC=0, NOPRINT;