Do not use this file for anchoring. ISFILE=filename produces an output file containing the category structure measure information for each item. All measures are added to the corresponding item's calibration and rescaled by USCALE= and UDECIMALS=. This file contains 4 heading lines (unless HLINES=N), followed by one line for each item containing:
Columns:
| Start | End Format Description |
| 1 | 1 A1 Blank or ";" if no responses or deleted (status = -2, -3) |
| 2 | 6 I5 1. The item sequence number (ENTRY) |
| 7 | 11 I5 2. The item's status (STATUS) |
1 = Estimated calibration
2 = Anchored (fixed) calibration
0 = Extreme minimum (estimated using EXTRSC=)
-1 = Extreme maximum (estimated using EXTRSC=)
-2 = No responses available for calibration
-3 = Deleted by user
| 12 | 5 I5 3. Number of active categories (MAXIMUM) |
| 17 | 5 I5 4. Lowest active category number (CAT) |
| 22 | 29 F8.2 5. Measure for an expected score of LOWADJ= (E.G., CAT+.25) |
The following fields are repeated for the remaining active categories:
| 30 | 34 I5 6. Active category number (CAT) |
| 35 | 39 I5 7. Ordered category number in structure (STRUCTURE) = "Step counting from zero" |
| 40 | 47 F8.2 8. Structure measure (MEASURE) = Rasch-Andrich threshold + item measure = Dij, the Rasch-Andrich threshold. The number of decimal places is set by UDECIMAL=. Do not use for anchoring. Use the SFILE= and IFILE=. |
| 48 | 55 F8.2 9. Rasch-Andrich threshold's standard error (ERROR) - reported if only one item in the ISGROUPS=, otherwise 0.0. |
| 56 | 63 F8.2 10. Measure for an expected score of category — 0.5 score points (CAT-0.5). This is the Rasch-half-point threshold, the boundary between categories when conceptualized as average performances. It is not a model parameter. |
| 64 | 71 F8.2 11. Measure for an expected score of category score points (AT CAT). This is the measure corresponding to a category when predicting for an individual or sample about which nothing else is known. For the top category this value corresponds to the top category value less HIADJ= (e.g., CAT-0.25), the measure for an expected score of HIADJ= score points less than the top category value. |
| 72 | 79 F8.2 12. Measure at the 50% cumulative probability (50%PRB). This is the Rasch-Thurstone threshold. These can be used as parameter estimates for a Rasch-Thurstone model. They are not parameter estimates of Leon Thurstone's "normal" model, nor of Samejima's "graded response" model. |
The "AT CAT" for the extreme categories are actually at -infinity and +infinity. So the value LOWADJ= above minimum score on the item substitutes for -infinity. For the extreme high category, the value HIADJ= less than the maximum score for that item substitutes for +infinity. It is these values that are plotted on Table 2.2 for the extreme categories.
Since the ISFILE= has the same number of category entries for every item, the repeated fields are filled out with "0" for any further categories up to the maximum categories for any item.
The format descriptors are:
In = Integer field width n columns
Fn.m = Numeric field, n columns wide including n-m-1 integral places, a decimal point and m decimal places
An = Alphabetic field, n columns wide
When CSV=Y, commas separate the values with quotation marks around the "Item name". When CSV=T, the commas are replaced by tab characters.
When STKEEP=YES and there are intermediate null categories, i.e., with no observations, then the structure calibration into the category is set 40 logits above the previous calibration. The structure calibration out of the category, and into the next category, is set 40 logits above. Thus:
Category structure Calibration
Table 3.2 In SFILE
0 NULL 0.00
1 -1.00 -1.00
2 NULL 39.00
3 1.00 -38.00
TOTAL: 0.00 0.00
Meanings of the columns
There are several ways of conceptualizing the category boundaries or thresholds of a rating (or partial credit) scale item. Imagine a rating (or partial credit) scale with categories, 1, 2, 3:
From the "expected score ogive", also called the "model item characteristic curve"
Average rating: Measure (must be ordered)
1.25 Measure for an expected score of 0.25 (CAT+.25) when LOWADJ=0.25
1.5 Measure for an expected score of category — 0.5 score points (CAT-0.5)
2 Measure for an expected score of category score points (AT CAT)
2.5 Measure for an expected score of category — 0.5 score points (CAT-0.5)
2.75 Measure for an expected score of category score points (AT CAT)
since this is the top extreme category the reported values is for CAT-0.25 when HIADJ=0.25
From the "category probability curves" relative to the origin of the measurement framework (need not be ordered)
1-2 equal probability Structure measure = Rasch-Andrich threshold + item measure = Dij (MEASURE)
standard error Rasch-Andrich threshold's standard error (ERROR)
2 maximum probability Measure for an expected score of category score points (AT CAT) - (yes, same as for the ogive)
2-3 equal probability Structure measure = Rasch-Andrich threshold + item measure = Dij (MEASURE)
standard error Rasch-Andrich threshold's standard error (ERROR)
From the "cumulative probability curves" (preferred by Thurstone) (must be ordered)
Category 1 at .5 probability Measure at the 50% cumulative probability (50%PRB)
Category 1+2 at .5 probability Measure at the 50% cumulative probability (50%PRB)
Example 1: You wish to write a file on disk called "ITEMST.FIL" containing the item statistics reported in Table 2.2, for use in constructing your own tables:
ISFILE = ITEMST.FIL
ISGROUPS = 0 ; each item has its own "partial credit" scale
LOWADJ = 0.25 ; the standard for the low end of the rating scale
HIADJ = 0.25 ; the standard for the high end of the rating scale
;ENTRY STAT MAX CAT CAT+.25 CAT STRU MEASURE ERROR CAT-0.5 AT CAT 50%PRB CAT STRU MEASURE ERROR CAT-0.5 CAT-.25 50%PRB
1 1 2 0 -2.47 1 1 -1.25 .00 -1.58 -.40 -1.40 2 2 .46 .00 .79 1.68 .61
2 1 2 0 -2.78 1 1 -1.57 .00 -1.89 -.71 -1.71 2 2 .15 .00 .48 1.37 .29
"ENTRY" is the item entry number
"STAT" is the item status, see IFILE=
"MAX" is the highest category
"CAT" is the current category
"CAT+0.25" is the measure corresponding to an expected score of the lowest category+0.25 score points on the item
"STRU" (structure calibration) or step measure is a Rasch model parameter estimate (Rasch-Andrich thresholds), also the point at which adjacent categories are equally probable. See "Category probability curves" graph.
"MEASURE" is the item difficulty + structure calibration.
"ERROR" is an estimate of the standard error. It is reported as .00 if it is not known.
"CAT-0.5" is the location where the expected score on the item is the category half-point value, e.g., for a scale for 0,1,2 the "CAT-0.5" values correspond to expected scores of 0.5 and 1.5. See the "Expected score ICC" graph.
"AT CAT" is the location where the expected score on the item is the category point value, e.g., for a scale for 0,1,2 the "At step" values correspond to expected scores of 0.25, 1, and 1.75. Since the "at step" values 0 and 2 are infinite they are reported for 0.25 and 1.75. See the "Expected score ICC" graph.
"50%PRB" is the location of the Rasch-Thurstone threshold, the point at which the probability of all categories below = the probability of all categories at or above. See the "Cumulative probabilities" graph.
Example 2: To produce a Table of expected measures per item-category similar to Pesudovs, K., E. Garamendi, et al. (2004). "The quality of life impact of refractive correction (QIRC) questionnaire: Development and validation." Optometry and Vision Science 81(10): 769-777, write the ISFILE= to Excel. Then delete or hide unwanted columns.
