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Table 2 Overall Rasch model fit statistics and reliability of the NHPD

From: Cross-diagnostic validity of the Nottingham health profile index of distress (NHPD)

  Original NHPD NHPD adjusted for DIF d
Item fit residual   
   Mean a -0.571 -0.488
   SD b 1.416 1.250
Total item-trait interaction   
   Chi-square (df) 134.337 (120) 188.538 (155)
   P-value 0.175 0.034
Reliability c 0.841 0.844
  1. a Should be close to 0 [35].
  2. b Should be close to 1 [35].
  3. c Index of person separation, a Rasch based reliability statistic analogous to Cronbach's alpha/KR-20 [22, 35]. Indicates the degree to which people can be separated into discrete groups. Values of 0.7 and 0.8 are the minimum required to discern two and there groups, respectively [44].
  4. d Items 4, 6, 7, 8, 11, 17 and 18 split by diagnosis.
  5. NHPD, Nottingham Health Profile index of Distress; DIF, differential item functioning; SD, standard deviation; df, degrees of freedom.