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Table 6 Distribution of scores and summary Classical Test Theory (CTT) and Rasch analysis results for the NHP20 and NHP22

From: Classical test theory versus Rasch analysis for quality of life questionnaire reduction

  NHP20 NHP22
  Total score Emotional Physical Sleep Pain Total score Physical Psychological
Number of items 20 7 5 4 4 22 11 11
Principal components results         
Loadings of the first component* - 0.70 0.68 0.60 0.72 - 0.92 0.92
Distribution of scores         
Valid observations 7,243 7,382 7,442 7,455 7,452 7,559 7,558 7,557
Mean 35.36 30.45 44.72 40.62 28.24 31.04 29.42 28.21
Standard deviation 29.33 31.46 38.10 39.34 36.50 23.84 28.06 27.40
25th Percentile 10 0 0 0 0 8.77 0 0
50th Percentile 30 14.29 40 25 0 28.52 23.56 24.22
75th Percentile 55 57.14 80 75 50 46.94 48.18 46.46
% 0 score 10.8 30.1 27.0 33.0 49.9 15.7 28.1 26.9
% 100 score 2.3 5.5 17.3 20.3 12.0 1.7 3.0 3.0
CTT analysis results         
Item-total correlation (range) 0.45–0.65 0.51–0.62 0.57–0.71 0.51–0.75 0.65–0.68 0.46–0.65 0.47–0.68 0.47–0.64
Reliability         
   Cronbach's α - 0.82 0.83 0.88 0.87 - 0.88 0.87
   Linear combination 0.94 - - - - 0.93 - -
Rasch analysis results         
Person separation 2.17 0.74 0.32 0.00 0.00 2.08 1.39 1.24
Person reliability 0.82 0.35 0.09 0.00 0.00 0.81 0.66 0.61
  1. * One component accounts of 67.5% of the total variance for the NHP20, and 85% of the total variance for the NHP22.