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Table 3 Item fit statistics and differential item functioning (DIF) for age and gender

From: Structural validity of the Boston Carpal Tunnel Questionnaire and its short version, the 6-Item CTS symptoms scale: a Rasch analysis one year after surgery

CTS-6 Location Fit residual DF Chi-square P DIF Gender (U/NON-U) DIF Age (U/NON-U)
Item
 How severe are the following symptoms in your hand?
  1. Pain at night 0,068 -1,212 123,67 4,006 0,26 - -
  2. Pain during daytime 1,209 1,671 123,67 1,773 0,62 - -
  3. Numbness or tingling at night -0,752 -0,524 123,67 1,749 0,62 - P=0.0498
  4. Numbness or tingling during daytime -0,441 0,520 123,67 3,048 0,38 - -
 How often did the following symptoms in your hand wake you up at night?
  5. Pain 0,239 -0,220 123,67 1,381 0,71 - -
  6. Numbness or tingling -0,323 -1,057 123,67 3,249 0,36 (P=0.0008) -
BCTQ Symptom Severity Scale Location Fit residual DF Chi-square P DIF Gender (U/NON-U) DIF Age (U/NON-U)
Item
 1. How severe is the hand or wrist pain that you have at night? 0,598 -0,879 167,00 1,893 0,60 - -
 2. How often did hand or wrist pain wake you up during a typical night in the past two weeks? 2,133 1,745 167,00 3,011 0,39 - -
 3. Do you typically have pain in your hand or wrist during the daytime? 0,373 -2,112 167,00 7,312 0,06 - -
 4. How often do you have hand or wrist pain during daytime? -0,78 -0,929 167,00 6,736 0,08 - -
 5. How long on average does an episode of pain last during the daytime? -1,112 -0,232 167,00 9,340 0,03 - NON-U; P=0.020
 6. Do you have numbness (loss of sensation) in your hand? -0,290 1,089 167,00 9,340 0,13 - -
 7. Do you have weakness in your hand or wrist? -0,915 1,507 167,00 7,979 0,05 U; P=0.003 -
 8. Do you have tingling sensations in your hand? -0,141 -0,648 167,00 1,421 0,70 - -
 9. How severe is numbness (loss of sensation) or tingling at night? -0,007 -1,604 167,00 4,850 0,18 - -
 10. How often did hand numbness or tingling wake you up during a typical night during the past two weeks? 0,510 -0,917 167,00 3,590 0,31 - -
 11. Do you have difficulty with the grasping and use of small objects such as keys or pens? -0,369 1,818 167,00 6,739 0,08 - -
BCTQ Functional Status Scale Location Fit residual DF Chi-square P DIF Gender (U/NON-U) DIF Age (U/NON-U)
Item
 1. Writing 1,454 0,140 137,88 6,934 0,07 -  
 2. Buttoning of clothes -0,591 -0,188 137,88 2,659 0,45 U; P=0.020 U; P<0.001
 3. Holding a book while reading -0,997 0,985 137,88 2,626 0,45 - -
 4. Gripping of a telephone handle 1,462 -0,542 137,88 3,175 0,37 NON-U; P=0.0007 U; P=0.0002
 5. Opening of jars -1,623 1,471 137,88 3,585 0,31 - U; P<0.001
 6. Household chores -0,477 -1,555 137,88 6,488 0,09 NON-U; P=0.020 -
 7. Carrying of grocery bags -0,887 -0,094 137,88 2,540 0,47 - U; P=0.004
 8. Bathing and dressing 1,659 -1,660 137,88 6,785 0,08 - U; P=0.020
  1. CTS-6 The 6-item CTS symptoms scale, DF degrees of freedom, P P-value, DIF differential item functioning, U uniform, NON-U non-uniform, BCTQ Boston Carpal Tunnel Questionnaire