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Table 3 Measurement properties of the Swedish CORE-OM, and alternative item sets, in a cohort of depressed and anxious patients

From: Measurement properties of the Swedish clinical outcomes in routine evaluation outcome measures (CORE-OM): Rasch analysis and short version for depressed and anxious out-patients in a multicultural area

Rasch statistic Investigated properties/Statistical question Fit criteria and interpretation CORE-OM [34 items] CORE-OM non risk [28 items] CORE-10 [10 items] CORE-6D emotional [5 items] CORE-6D added [8 items]
Person separation index Are item responses consistent across respondents? Values > 0.8 individual use, > 0.7 group use 0.95 0.94 0.86 0.73 0.85
Overall model fit, mean (SD) Do observed items responses correlate with expected responses from the Rasch model? Perfect fit = mean of 0 and SD of 1 Acceptable fit = SD < 1.5 0.41 (4.00) 0.63 (4.44) 0.20 (3.69) − 0.10 (1.49) − 0.02 (1.41)
Item-trait interaction, chi-square p-value Probability that the overall responses fit the model? Non-significant Bonferroni-adjusted p-value 917.53 p < 0.001 839.78 p < 0.001 278.23 p < 0.001 56.78 p = 0.0113* 76.11 p = 0.038*
Individual item fit Do the observed individual item responses correlate with expected responses from the Rasch model? Fit residual =  + / − 2.5 Chi-square p-values nonsignificant (Bonferroni-adjusted). Visual check of item characteristic curves 9 items show misfit and sign p-value 9 items show misfit and sign p-value 5 item show misfit and sign p-value No item misfit No item misfit
Differential item functioning Does any item deviate from the requirement of invariance across groups for gender, age and diagnosis? Nonsignificant Bonferroni-adjusted probability value DIF gender items 4, 19; age items 8, 9; time items 2, 8, 19, 23, 27 DIF gender items12, 16; DIF age items7, 16 DIF age item 6 DIF time items 1, 8 DIF age items 5, 16 DIF age item 5
Local dependency Does any item show dependency on a response to another item? No correlations above the relative cut off greater than 0.20 above the average correlations 27 (of 561) correlation (relative cut-off 0.17) 24 (of 351) correlations (relative cutoff 0.17) 26 (of 45) correlation (relative cutoff 0.1) None None
Thresholds Do the response categories work as intended, or are there disordered thresholds? Ordered thresholds 7 disordered thresholds, items 6, 8, 9, 16, 22, 31, 34 1 disordered threshold, item 8 1 disordered threshold, item 16 1 disordered threshold, item 16 1 disordered threshold, item 16
Unidimensionality Does the questionnaire measure one single dimension? The proportion of t tests reaching significance should not exceed 5% in the independent t test protocol 20.9% 21.9% 7.29% 1.15% 4.61%
  1. *The Bonferroni-adjusted alpha level (0.05/number of items) was 0.01 for CORE-6D and 0.00625 for CORE-6D added