Rasch estimates are constructed to be as sample independent as is statistically possible, but you must still take care to maintain comparability of measures across analyses. For instance, if a rating scale or partial credit structure is used, and a high-low measure split is made, then the low rating scale (or partial credit) categories may not appear in the data for the high measure sample and vice versa. To compare item calibrations for the two samples requires the response structure to be calibrated on both samples together, and then the response structure calibrations to be anchored for each sample separately. Comparison of patient measures from separate analyses requires both the response structure calibrations and the item calibrations to share anchor calibrations. 35 arthritis patients have been through rehabilitation therapy. Their admission to therapy and discharge from therapy measures are to be compared. They have been rated on the 13 mobility items of the Functional Independence Measure (FIM™). Each item has seven levels. At admission, the patients could not perform at the higher levels. At discharge, all patients had surpassed the lower levels (Data courtesy of C.V. Granger & B. Hamilton, ADS). A generic control file is in EXAM12.TXT. The admission ratings are in EXAM12LO.TXT and the discharge ratings in EXAM12HI.TXT. Three analyses are performed: 1) joint analysis of the admission (low) and discharge (high) data to obtain response structure calibrations, 2 & 3) separate runs for the admission (low) and discharge (high) data to obtain item calibrations. For a more complex situation, see Example 17.
; This common control file is EXAM12.TXT
TITLE='GENERIC ARTHRITIS FIM CONTROL FILE'
ITEM1=7 ; Responses start in column 7
NI=13 ; 13 mobility items
CODES=1234567 ; 7 level rating scale
CLFILE=* ; Defines the rating scale
1 0% Independent
2 25% Independent
3 50% Independent
4 75% Independent
5 Supervision
6 Device
7 Independent
*
&END
A. EATING
B. GROOMING
C. BATHING
D. UPPER BODY DRESSING
E. LOWER BODY DRESSING
F. TOILETING
G. BLADDER
H. BOWEL
I. BED TRANSFER
J. TOILET TRANSFER
K. TUB, SHOWER
L. WALK/WHEELCHAIR
M. STAIRS
END NAMES
The admission data is in file EXAM12LO.TXT:
21101 5523133322121 Patient number in cols 1-5, ratings in 7-19
21170 4433443345454
|
22618 4433255542141
22693 3524233421111
The discharge data is in file EXAM12HI.TXT:
21101 5734366655453 Ratings generally higher than at admission
21170 6466677777676
|
22618 7667656666565
22693 7776677676677
The batch file to run this is (see BATCH=), under Windows XP, 2000, EXAM12.CMD:
REM COMPARISON OF ITEM CALIBRATIONS FOR HIGH AND LOW SAMPLES
START /WAIT ..\WINSTEPS EXAM12.TXT EXAM12OU.TXT DATA=EXAM12LO.TXT+EXAM12HI.TXT TITLE=ADMIT+DISCHARGE SFILE=EXAM12SF.TXT BATCH=Y
START /WAIT ..\WINSTEPS EXAM12.TXT EXAM12LU.TXT DATA=EXAM12LOF.TXT TITLE=ADMIT SAFILE=EXAM12SF.TXT IFILE=EXAM12LI.TXT BATCH=Y
START /WAIT ..\WINSTEPS EXAM12.TXT EXAM12HU.TXT DATA=EXAM12HIF.TXT TITLE=DISCHARGE SAFILE=EXAM12SF.TXT IFILE=EXAM12HI.TXT BATCH=Y
under WINDOWS-98, EXAM12BAT.BAT: (not Windows-9 nor 3.1)
REM COMPARISON OF ITEM CALIBRATIONS FOR HIGH AND LOW SAMPLES
START /w ..\WINSTEPS EXAM12.TXT EXAM12OU.TXT DATA=EXAM12LO.TXT+EXAM12HI.TXT TITLE=ADMIT+DISCHARGE SFILE=EXAM12SF.TXT BATCH=Y
START /w ..\WINSTEPS EXAM12.TXT EXAM12LU.TXT DATA=EXAM12LO.TXT TITLE=ADMIT SAFILE=EXAM12SF.TXT IFILE=EXAM12LIF.TXT BATCH=Y
START /w ..\WINSTEPS EXAM12.TXT EXAM12HU.TXT DATA=EXAM12HI.TXT TITLE=DISCHARGE SAFILE=EXAM12SF.TXT IFILE=EXAM12HIF.TXT BATCH=Y
Under WINDOWS-NT (early versions), EXAM12NT.BAT:
REM COMPARISON OF ITEM CALIBRATIONS FOR HIGH AND LOW SAMPLES
..\WINSTEPS EXAM12.TXT EXAM12OU.TXT DATA=EXAM12LO.TXT+EXAM12HI.TXT TITLE=ADMIT&DISCHARGE SFILE=EXAM12SF.TXT BATCH=Y
..\WINSTEPS EXAM12.TXT EXAM12LU.TXT DATA=EXAM12LO.TXT TITLE=ADMIT SAFILE=EXAM12SF.TXT IFILE=EXAM12LIF.TXT BATCH=Y
..\WINSTEPS EXAM12.TXT EXAM12HU.TXT DATA=EXAM12HI.TXT TITLE=DISCHARGE SAFILE=EXAM12SF.TXT IFILE=EXAM12HIF.TXT BATCH=Y
To run this, select "Run batch file" from "Batch" pull-down menu, and right-click on "Exam12bat.bat" or "Exam12cmd.cmd" in the dialog box, then left-click on "open".
The shared structure calibration anchor file is EXAM12SF.TXT:
; structure measure FILE FOR
; ADMIT&DISCHARGE
; May 23 13:56 1993
; CATEGORY structure measure
1 .00
2 -2.11
3 -1.61
4 -1.25
5 .06
6 1.92
7 2.99
The item calibrations measures for admission and discharge are written into IFILE= files, with comma-separated values (CSV=Y), so that they can easily be imported into a spreadsheet.