SFILE= structure-threshold output file

If SFILE=filename is specified, a file is output which contains the item and category information needed for anchoring structures. It has 4 heading lines (unless HLINES=N), and has the format:

 

Example for dichotomous items:

; STRUCTURE-THRESHOLD MEASURE ANCHOR FILE

; CATEGORY  Rasch-Andrich threshold

 1     .00 ; this is a dummy threshold to tell Winsteps that 0 is the bottom category

 2     .00

 

Example for the Andrich Rating Scale model:

; STRUCTURE-THRESHOLD MEASURE ANCHOR FILE

; CATEGORY  Rasch-Andrich threshold

 0     .00 ; this is a dummy threshold to tell Winsteps that 0 is the bottom category

 1    -.86

 2     .86

 

Example for the Partial Credit model:

STRUCTURE-THRESHOLD MEASURE ANCHOR FILE

; ACT CATEGORY  Rasch-Andrich threshold MEASURE

    1  0     .00 ; this is a dummy threshold to tell Winsteps that 0 is the bottom category

    1  1   -1.64

    1  2    1.64

    2  0     .00 ; this is a dummy threshold to tell Winsteps that 0 is the bottom category

    2  1    -.30

    2  2     .30

    3  0     .00        ; this is a dummy threshold to tell Winsteps that 0 is the bottom category

    3  1   -1.13

    3  2    1.13

....

 

Columns:

1. The item sequence number (I6), if required.

Shown only if there are multiple groupings in ISGROUPS= or SAITEM=Yes, otherwise this is omitted.

 

2. The category number (I3) (STRU)  - the higher numbered of the pair of categories on each side of the Andrich threshold.

 

3. Andrich threshold value (structure calibration) (F7.2) (user-rescaled by USCALE=) (number of decimals by UDECIM=) (MEASURE)

Use this for anchoring, together with IFILE=

 

If CSV=Y, these values are separated by commas. When CSV=T, the commas are replaced by tab characters.

 

SFILE=? opens a Browse window

 


 

Andrich Thresholds for Unobserved Categories

 

When STKEEP=YES and there are intermediate null categories, i.e., with no observations, then the Rasch-Andrich thresholds are infinite. To approximate infinite Andrich thresholds in computer arithmetic, the Andrich threshold into the unobserved category is set 40 logits above the highest threshold calibration. The Rasch-Andrich threshold out of the unobserved category, and into the next category, is set down by the same amount. Thus:

 

Category Structure Calibration
Rasch-Andrich Threshold

Category

Table 3.2

In SFILE

0

NULL

0

1

-3

-3

2

NULL

40 + 2 = 42

3

1

1 - 40 - 2 = -41

4

2

2

TOTAL:

0

0

 

This is also an attempt to model unobserved categories for the next time, when they may be observed.

 

If categories will not be observed next time, then please specify STKEEP=NO, which automatically drops the category from the rating (or partial credit) scale.

 

If categories may be observed next time, then it is better to  include a dummy data record in your data file which includes an observation of the missing category, and reasonable values for all the other item responses that accord with that missing category. This one data record will have minimal impact on the rest of the analysis.


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Rating Scale Analysis, Wright & Masters Introduction to Rasch Measurement, E. Smith & R. Smith Introduction to Many-Facet Rasch Measurement, Thomas Eckes Invariant Measurement with Raters and Rating Scales: Rasch Models for Rater-Mediated Assessments, George Engelhard, Jr. & Stefanie Wind Statistical Analyses for Language Testers, Rita Green
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar Journal of Applied Measurement Rasch models for measurement, David Andrich Constructing Measures, Mark Wilson Rasch Analysis in the Human Sciences, Boone, Stave, Yale
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