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) |
An example item number from the response-structure grouping - see ISGROUPS= |
6 |
MEAS (Measure) |
The measure relative to item difficulty (user-rescaled by USCALE=) with UDECIMALS= decimal places. Add the item difficulty to MEAS for absolute measure relative to the latent variable. The plotted range is at least MRANGE= away from its center. |
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 your item's category response curves and its item information function.
GRFILE=MYDATAGR.txt
With CSV=Y
"PROBABILITY CURVES FOR LIKING FOR SCIENCE (Wright & Masters p.18) Jul 4 16:03 2000"
;ITEM,MEAS,SCOR,INFO,0,1,2
1,-3.00,.11,.10,.89,.10,.00
1,-2.94,.12,.11,.89,.11,.00
1,-2.88,.12,.11,.88,.12,.00
1,-2.82,.13,.12,.87,.12,.00
1,-2.76,.14,.12,.87,.13,.00
1,-2.70,.14,.13,.86,.14,.00
With CSV=N (fixed spacing)
; PROBABILITY CURVES FOR LIKING FOR SCIENCE (Wright & Masters p.18) Nov 23 23:31 2008
ACT MEAS SCOR INFO 0 1 2
1 -3.00 .11 .10 .89 .10 .00
1 -2.94 .12 .11 .89 .11 .00
1 -2.88 .12 .11 .88 .12 .00
1 -2.82 .13 .12 .87 .12 .00
1 -2.76 .14 .12 .87 .13 .00
Example 2: I want the information function for every item in my test relative to the latent variable (absolute x-axis). All my items share the same rating scale.
1. output the IFILE= to Excel
2. delete all columns except the item difficulties
3. transpose the item difficulties into a row
4. output the GRFILE= to Excel
5. delete unwanted columns and unwanted top row
6. move MEAS into the first column, INFO in the second column.
7. copy the item difficulty row from the IFILE worksheet into the top row of the GRFILE= worksheet starting in the third column
7. in each cell of the (information x item difficulty) rectangle, add the item difficulty from row 1 to the MEAS value in column 1
8. for each item, plot the INFO (column 2, same for all items) against the item's difficulty (MEAS) column
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