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