GRFILE= probability curve coordinate output file

If GRFILE=filename is specified, a file is output which contains a list of measures relative to item difficulty (x-axis coordinates) and corresponding expected scores and category probabilities (y-axis coordinates) to enable you to use your own plotting program to produce item-category plots like those in the Graphs menu.

 

GRFILE=? opens a Browse window

 

Column Heading

Column Contents

Fixed Columns

ITEM (or ITEM= value)

Item number

6

MEAS (Measure without item difficulty added)

 

MEAS+ (Measure with item difficulty added)

IADD=No (default): The measure relative to item difficulty (user-rescaled by USCALE=) with UDECIMALS= decimal places.

The plotted range is at least MRANGE= away from its center.

IADD=Yes adds the item difficulty to MEAS for absolute measure relative to the latent variable.

7

SCOR (Score)

Expected score for the Modeled Item Characteristic Curve (ICC) or Item Response Function (IRF).

7

INFO (Information)

Statistical information for Item Information Function. Sum these for all items for the Test Information Function (TIF).

7

0 (Category number)

Probability of observing lowest category

7

1, 2, etc.  (Category numbers)

Probability of observing higher categories

7

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

 

For the numbers plotted for empirical ICCs, see the "Empirical ICC" plot, "Copy data to Clipboard"

 

Example 1: You wish to write a file on disk called "MYDATAGR.txt" containing x- and y-coordinates for plotting a generic item's category response curves and its item information function. Item difficulty is not added.

 

IADD=No

GRFILE=MYDATAGR.txt

 

With CSV=Y

 

"PROBABILITY CURVES FOR LIKING FOR SCIENCE (Wright & Masters p.18) Aug 13 2023 12:40"

"ITEM","MEASURE","SCORE","INFORMATION",0,1,2

1,-4.00,.0420,.0409,.9583,.0414,.0003

1,-3.99,.0423,.0412,.9580,.0417,.0003

1,-3.98,.0427,.0415,.9577,.0420,.0003

1,-3.98,.0430,.0418,.9573,.0423,.0003

1,-3.97,.0433,.0421,.9570,.0427,.0003

 

With CSV=N (fixed spacing)

 

; PROBABILITY CURVES FOR LIKING FOR SCIENCE (Wright & Masters p.18) Aug 13 2023 12:40

   ACT   MEAS   SCOR   INFO      0      1      2

     1  -4.00  .0420  .0409  .9583  .0414  .0003

     1  -3.99  .0423  .0412  .9580  .0417  .0003

     1  -3.98  .0427  .0415  .9577  .0420  .0003

     1  -3.98  .0430  .0418  .9573  .0423  .0003

     1  -3.97  .0433  .0421  .9570  .0427  .0003

 

Example 2: I want the information function for every item in my test relative to the latent variable (absolute x-axis).

 

IADD=Yes  ; item difficulty is added

GRFILE=MYDATAGR.txt

Columns: ITEM, MEAS, INFO

 

; PROBABILITY CURVES FOR LIKING FOR SCIENCE (Wright & Masters p.18) Aug 13 2023 12:40

   ACT  MEAS+   SCOR   INFO      0      1      2

     1  -7.40  .0021  .0021  .9979  .0021  .0000

     1  -7.38  .0022  .0022  .9978  .0022  .0000

     1  -7.37  .0022  .0022  .9978  .0022  .0000

     1  -7.36  .0022  .0022  .9978  .0022  .0000

     1  -7.34  .0023  .0023  .9977  .0023  .0000

 

Example 3:The Category Information Functions are the item information function multiplied by the category probability for each category for each measure. Use Excel for the calculation.

 

IADD=Yes  

GRFILE=MYDATAGR.xls

 

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