Table 13.
Item Response Theory (IRT) parameters for PHQ-9 items
| Item | a | b₁ | b₂ | b₃ |
|---|---|---|---|---|
| PHQ9_interest | 0.468 | −2.958 | 1.657 | 3.937 |
| PHQ9_down | 1.900 | −0.850 | 0.873 | 1.625 |
| PHQ9_sleep | 1.491 | −0.814 | 0.596 | 1.474 |
| PHQ9_energy | 1.562 | −1.320 | 0.313 | 1.271 |
| PHQ9_appetite | 1.783 | −0.601 | 0.684 | 1.423 |
| PHQ9_self_worth | 1.991 | −0.413 | 0.730 | 1.458 |
| PHQ9_concentration | 1.821 | −0.403 | 0.739 | 1.536 |
| PHQ9_motor | 1.621 | −0.028 | 1.122 | 1.931 |
| PHQ9_self_harm | 1.758 | 0.117 | 1.213 | 1.994 |
Graded Response Model (GRM) discrimination (a) and difficulty thresholds (b₁–b₃) for PHQ-9 items